Bottom gate tft with multilayer passivation

ABSTRACT

A transistor includes a gate in contact with a substrate. A gate insulating layer is in contact with at least the gate. An inorganic semiconductor layer is in contact with the gate insulating layer. There is a source electrode in contact with a first portion of the inorganic semiconductor layer and a drain electrode in contact with a second portion of the inorganic semiconductor layer, and the source electrode and the drain electrode are separated by a gap. There is a multilayer insulating structure in contact with at least the inorganic semiconductor layer in the gap. The multilayer structure includes an inorganic dielectric layer having a first pattern defining a first area; and a polymer structure having a second pattern defining a second area. The second area is located within the first area and the polymer structure is in contact with the semiconductor layer in the gap.

CROSS REFERENCE TO RELATED APPLICATIONS

Reference is made to commonly-assigned, U.S. patent application Ser. No.______ (Docket K001569), entitled “VERTICAL TFT WITH MULTILAYERPASSIVATION”, Ser. No. ______ (Docket K001764), entitled “TOP GATE TFTWITH POLYMER INTERFACE CONTROL LAYER”, Ser. No. ______ (Docket K001765),entitled “METHOD OF FORMING A PATTERNED POLYMER LAYER”, all filedconcurrently herewith.

FIELD OF THE INVENTION

This invention relates generally to patterned thin film fabrication andto electronic and optoelectronic devices including patterned thin filmmultilayer dielectric structures. In particular, this invention relatesto selective area deposition of materials including, for example,metal-oxides in combination with polymer dielectric materials anddevices including, for example, thin film transistors produced usingthis fabrication technique.

BACKGROUND OF THE INVENTION

Modern-day electronics require multiple patterned layers of electricallyor optically active materials, sometimes over a relatively largesubstrate. Electronics such as radio frequency identification (RFID)tags, photovoltaics, optical and chemical sensors all require some levelof patterning in their electronic circuitry. Flat panel displays, suchas liquid crystal displays or electroluminescent displays (for example,OLED), rely upon accurately patterned sequential layers to form thinfilm components of the backplane. These components include capacitors,transistors, and power buses. The industry is continually looking fornew methods of materials deposition and layer patterning for bothperformance gains and cost reductions. Thin film transistors (TFTs) areone common electronic component, and can serve to illustrate themanufacturing issues for many thin film components. TFTs are widely usedas switching elements in electronics, for example, in active-matrixliquid-crystal displays, smart cards, and a variety of other electronicdevices and components thereof.

In the past decade, various materials have received attention as apotential alternative to amorphous silicon for use in semiconductorchannels of thin film transistors. Semiconductor, dielectric,conducting, and protective materials that are simpler to process aredesirable, especially those that are capable of being applied to largeareas by relatively simple processes. The discovery of practicalinorganic semiconductors as a replacement for current silicon-basedtechnologies has also been the subject of considerable research efforts.For example, metal oxide semiconductors are known that constitute zincoxide, indium oxide, gallium indium zinc oxide, tin oxide, or cadmiumoxide deposited with or without additional doping elements includingmetals such as aluminum. Such semiconductor materials, which aretransparent, can have an additional advantage for certain applications.

A semiconductor material useful in a TFT must display severalcharacteristics. In typical applications of a thin film transistor, thedesire is for a switch that can control the flow of current through thedevice. As such, it is desired that when the switch is turned on a highcurrent can flow through the device. The extent of current flow isrelated to the semiconductor charge carrier mobility. When the device isturned off, it is desired that the current flow be very small. The ratiobetween current flow in the on state to current flow in the off state isrelated to the native charge carrier concentration. It is furtherdesired that the device remain unchanged during operation. The stabilityof transistors is typically evaluated by holding the device under aconstant stress (or bias) that is consistent with the stress applied tothe transistor in operation for a given application.

Many electronic devices benefit from the presence of either apassivation layer or a barrier layer or both. Thin film metal oxideTFTs, such as ZnO, GIZO, or GZO, have instabilities that can limit theiradoption in practical applications. There has been a concerted effortrecently to improve the stability of these types of TFTs withpassivation layers. Typical passivation layer structures employinorganic thin films as the passivation layer, such as Al₂O₃. The use ofthese inorganic passivation layers typically induces a negativethreshold shift that can be undesirable. Complicated processing schemeshave been introduced to passivate with inorganic materials withoutthreshold shifts. Alternatively, researchers have used multilayerchannels to modify the charge on the back channel, for instance usingtwo different stoichiometries of IGZO for the semiconductor layer. Therehas been limited work done to passivate inorganic TFTs withphotopattemable polymers, with varied response. In most cases, anegative shift in threshold voltage is still present with passivationand the processing involves the complex multistep process associatedwith photolithography and additional post deposition annealing steps.There remains a need for a passivation process for metal oxidetransistors which is simple, and which results in TFTs stable under biasstress without an associated shift in threshold shift from theunpassivated state.

Furthermore, it is recognized in the art that the material that is incontact with the back channel of a semiconductor has an impact on theperformance of the transistor. In the aforementioned cases, thepassivation layer is deposited on the back channel of a bottom gatedevice. In other architectures, controlling the back channel interfaceis still important even when the material layer does not impact theenvironmental stability of the device. For instance, in the case of topgate TFTs it has been observed that ZnO-based transistors built on glasshave very negative threshold voltages. There remains a need for devicestructures and material layers that control the back channel interfacein all types of device architectures including bottom gate transistors,top gate transistors, and vertical transistors.

Atomic layer deposition (ALD) can be used as a fabrication step forforming a number of types of thin-film electronic devices, includingsemiconductor devices and supporting electronic components such asresistors and capacitors, insulators, bus lines, and other conductivestructures. ALD is particularly suited for forming thin layers of metaloxides in the components of electronic devices. General classes offunctional materials that can be deposited with ALD include conductors,dielectrics or insulators, and semiconductors. Examples of usefulsemiconducting materials are compound semiconductors such as galliumarsenide, gallium nitride, cadmium sulfide, zinc oxide, and zincsulfide. A dielectric material is any material that is a poor conductorof electricity, often also referred to as an insulator material. Suchmaterials typically exhibit a bulk resistivity greater than 10¹⁰ Ω-cm.Examples of dielectrics are SiO2, HfO, ZrO, SiNx, and Al₂O₃.

There has been growing interest in combining ALD with a technology knownas selective area deposition (SAD). As the name implies, selective areadeposition involves treating portion(s) of a substrate such that amaterial is deposited only in those areas that are desired, or selected.These methods have been used to fabricate devices, as described in Appl.Phys. Lett. 2013, 103 (4), 043505 by Levy, et. al. There are manypotential advantages to selective area deposition techniques, such aseliminating an etch process for film patterning, reduction in the numberof cleaning steps required, and patterning of materials which aredifficult to etch. There, however, persists a problem of combiningmultiple SAD steps to form working devices, in combination with organicelements in a device. In some device architectures, the removal of theinhibitor used in selective area deposition can damage or otherwisenegatively impact the underlying layer. There remains a need for methodsto fabricate devices with organic elements as well as SAD methods thatdo not require the removal of the deposition inhibitor.

Accordingly, there still remains a need for high-quality passivation andback channel control layers that result in stable, high-quality devicesand that can be formed with simple processing methods. Correspondingly,a method is needed to simply pattern this layer for easy deviceintegration. Furthermore there is a need for novel processes for formingmultilayer insulating structures using selective area processes.

SUMMARY OF THE INVENTION

According to an aspect of the invention, a transistor includes asubstrate and a gate in contact with the substrate. A gate insulatinglayer is in contact with at least the gate and an inorganicsemiconductor layer is in contact with the gate insulating layer. Thereis a source electrode in contact with a first portion of the inorganicsemiconductor layer and a drain electrode in contact with a secondportion of the inorganic semiconductor layer, and the source electrodeand the drain electrode are separated by a gap. There is a multilayerinsulating structure in contact with at least the inorganicsemiconductor layer in the gap. The multilayer structure includes aninorganic dielectric layer having a first pattern defining a first area;and a polymer structure having a second pattern defining a second area.The second area is located within the first area and the polymerstructure is in contact with the semiconductor layer in the gap.

BRIEF DESCRIPTION OF THE DRAWINGS

In the detailed description of the example embodiments of the inventionpresented below, reference is made to the accompanying drawings, inwhich:

FIGS. 1 a and 1 b are a cross-sectional view and plan view,respectively, of one embodiment of a thin film transistor having amultilayer insulating structure of the present invention;

FIGS. 2 a, 2 c, 2 e, 2 g, 2 i, 2 k, 2 m, and 2 o are cross sectionalviews of the plan views of FIGS. 2 b, 2 d, 2 f, 2 h, 2 j, 2 n and 2 prespectively, illustrating various embodiments of the multilayerinsulating structure of the present invention;

FIG. 3 is another embodiment of a multilayer insulating structure of thepresent invention having two polymer layers used to passivate a bottomgate TFT;

FIG. 4 is another embodiment of a multilayer insulating structure of thepresent invention used to passivate a bottom gate TFT;

FIGS. 5 a and 5 c are cross-sectional views of the plan view of FIG. 5 bthat illustrate embodiments of the present invention multilayerinsulating structure having two inorganic insulating layers;

FIG. 6 a is a cross-sectional view of the plan view of FIG. 6 billustrating an embodiment of the present invention having multilayerinsulating structure with an inorganic insulating layer that covers anarea greater than a single TFT;

FIGS. 7 a and 7 b are cross-sectional views and plan views,respectively, of an embodiment of a dual-gate thin film transistorhaving a multilayer insulating structure of the present invention;

FIGS. 8 a and 8 b are a cross-sectional view and plan view,respectively, of an embodiment of a vertical thin film transistor havinga multilayer insulating structure of the present invention as apassivation layer;

FIGS. 9 a, 9 b and 9 c illustrate alternative geometries of gatestructures for vertical transistors of the present invention;

FIG. 10 is another embodiment of a multilayer insulating structure ofthe present invention having two polymer layers used to passivate avertical TFT;

FIG. 11 is another embodiment of a multilayer insulating structure ofthe present invention having two inorganic insulating layers used topassivate a vertical TFT;

FIG. 12 is another embodiment of a multilayer insulating structure ofthe present invention having two polymer layers and two inorganicinsulating used to passivate a vertical TFT;

FIGS. 13 a and 13 b are a cross-sectional view and plan view,respectively, of a top-gate thin film transistor formed over a uniformpolymer layer;

FIGS. 14 a and 14 b are a cross-sectional view and plan view,respectively, of a top-gate thin film transistor of the presentinvention formed over a polymer layer having the same pattern as thesemiconductor layer;

FIGS. 15 a and 15 b are a cross-sectional view and plan view,respectively, of a top-gate thin film transistor of the presentinvention formed over a polymer layer having a different pattern fromthe pattern of the semiconductor;

FIG. 16 is a flow chart describing the steps of one embodiment of thepresent process for forming a patterned polymer structure;

FIGS. 17 a through 17 d are cross-sectional side views of the steps ofone embodiment of the present process for forming a patterned multilayerinsulating structure;

FIG. 18 is a cross-sectional side view of a deposition device used forthin film deposition in the Examples;

FIG. 19 is a graph showing the change in threshold voltage with biasstress for Comparative Examples C1, C2, and C3 and Inventive Example I1;

FIG. 20 is a graph showing performance I_(ds)−V_(g) curvecharacteristics from the transistors of Comparative Example C3 andInventive Example I1 as measured with V_(d)=0.2 Volt;

FIG. 21 is a graph showing performance I_(ds)−V_(g) curvecharacteristics from a transistor of Inventive Example I2 as measuredwith V_(d)=0.2 Volt;

FIG. 22 is a graph showing performance I_(ds)−V_(g) curvecharacteristics from a transistor of Inventive Example I3 as measuredwith V_(d)=0.2 Volt; and

FIG. 23 is a graph showing performance I_(ds)−V_(g) curvecharacteristics in the saturation regime from the transistors ofComparative Examples C4 and C5, and Inventive Example I4.

DETAILED DESCRIPTION OF THE INVENTION

The following description is directed in particular to elements formingpart of, or cooperating more directly with, apparatus in accordance withthe present invention. It is to be understood that elements notspecifically shown or described may take various forms well known tothose skilled in the art. In the following description and drawings,identical reference numerals have been used, where possible, todesignate identical elements. It is to be understood that elements andcomponents can be referred to in singular or plural form, asappropriate, without limiting the scope of the invention.

The example embodiments of the present invention are illustratedschematically and not to scale for the sake of clarity. One of theordinary skills in the art will be able to readily determine thespecific size and interconnections of the elements of the exampleembodiments of the present invention.

Throughout the specification and claims, the following terms take themeanings explicitly associated herein, unless the context clearlydictates otherwise. The meaning of “a,” “an,” and “the” includes pluralreference, the meaning of “in” includes “in” and “on.” Additionally,directional terms such as “on”, “over”, “top”, “bottom”, “left”, “right”are used with reference to the orientation of the Figure(s) beingdescribed. Because components of embodiments of the present inventioncan be positioned in a number of different orientations, the directionalterminology is used for purposes of illustration only and is in no waylimiting.

A stable enhancement-mode transistor is a transistor in which there isnegligible off-current flow relative to on-current flow between a sourceand a drain at zero gate voltage. In other words, the transistor deviceis normally off. In contrast, a depletion-mode transistor is normally onmeaning that more than substantially negligible current flows between asource and a drain at zero gate voltage.

When the TFT operates in an enhancement-mode, the charges injected fromthe source electrode into the semiconductor are mobile and a currentflows from source to drain, mainly in a thin channel region within about100 Angstroms of the semiconductor-dielectric interface. See A.Dodabalapur, L. Torsi H. E. Katz, Science 1995, 268, 270, herebyincorporated by reference. In the absence of a gate field the channelideally has few charge carriers; as a result there is ideally nosource-drain conduction when the device is in off mode.

The off-current in an enhancement-mode device is defined as the currentflowing between the source electrode and the drain electrode when chargehas not been intentionally injected into the channel by the applicationof a gate voltage. This occurs for a gate-source voltage more negative,assuming an n-channel device, than a certain voltage known as thethreshold voltage. See Sze in Semiconductor Devices—Physics andTechnology, John Wiley & Sons (1981), pages 438-443, hereby incorporatedby reference. The on-current is defined as the current flowing betweenthe source electrode and the drain electrode when charge carriers havebeen accumulated intentionally in the channel by application of anappropriate voltage to the gate electrode and the channel is conducting.For an n-channel accumulation-mode TFT, this occurs at a gate-sourcevoltage more positive than the threshold voltage. It is desirable forthis threshold voltage to be slightly positive, for n-channel operation.Switching between on and off states is accomplished by the applicationand removal of an electric field from the gate electrode across the gatedielectric to the semiconductor-dielectric interface, effectivelycharging a capacitor.

Attractive TFT device characteristics include a high ratio of theon-current to the off-current, and a steep sub-threshold slope. In theoperation of such a TFT device, a voltage applied between the source anddrain electrodes establishes a substantial current flow only when thecontrol gate electrode is energized. That is, the flow of currentbetween the source and drain electrodes is modulated or controlled bythe bias voltage applied to the gate electrode. The relationship betweenmaterial and device parameters of the zinc-oxide-based semiconductor TFTcan be expressed by the approximate equation (see Sze in SemiconductorDevices—Physics and Technology, John Wiley & Sons (1981)):

$I_{d} = {\frac{W}{2L}\mu \; {C_{ox}\left( {V_{g} - V_{th}} \right)}^{2}}$

where I_(d) is the saturation source-drain current, C_(ox) is thegeometric gate capacitance associated with the insulating layer, W and Lare physical device dimensions, μ is the carrier mobility in thezinc-oxide-based semiconductor, V_(g) is the applied gate voltage, andV_(th) is the threshold voltage. Ideally, the TFT allows passage ofcurrent only when a gate voltage of appropriate polarity is applied.However, with zero gate voltage, the off current between source anddrain depends on the intrinsic conductivity a of the zinc-oxide-basedsemiconductor as well as the state of the semiconductor back channel. Ithas been found that for metal oxide transistors the interface of thesemiconductor opposite the gate, often called the back channel, plays arole in the device performance. The multilayer insulating structure ofthe present invention is useful to ensure that the metal oxidetransistors operate in enhancement mode. Additionally, it is known thatmetal oxide transistors can be sensitive to environmental conditions.The multilayer insulating structures of the present invention also serveto protect the transistors from environmental conditions, thereby bothpassivating the device and providing barrier properties.

The TFT structures described herein include an inorganic semiconductorchannel, preferably a metal-oxide-based semiconductor, with conductingelectrodes, commonly referred to as a source and a drain, for injectinga current into the semiconductor and a capacitively coupled gate forcontrolling and/or modulating the source-drain current. Preferred metaloxide based semiconductors include ZnO based semiconductors. Examples ofZnO based semiconductors are ZnO, InZnO, InO, and InGaZnO and doped ZnO.

The production of inorganic thin film transistors and electronic devicesfrom semiconductor, dielectric, conductive materials can be accomplishedby conventional techniques known to the skilled artisan. The exactprocess sequence is determined by the structure of the desiredtransistor. Thus, in the production of a field effect transistor in aso-called inverted structure, a gate electrode can be first deposited ona substrate, for example a vacuum- or solution-deposited metal ororganic conductor, or an ALD-deposited conductor. The gate electrode isinsulated with a dielectric and the source and drain electrodes and alayer of the inorganic semiconductor material are applied on top. Thestructure of such a transistor, and hence the sequence of itsproduction, are varied in the customary manner known to a person skilledin the art. Alternatively, a gate electrode is deposited first, followedby a gate dielectric, the semiconductor is applied, and finally thecontacts for the source electrode and drain electrode are deposited onthe semiconductor layer. In an alternative third structure, often calleda top-gate structure, the source and drain electrodes are depositedfirst, then the semiconductor is deposited, and finally the dielectricand gate electrode are deposited on top. In yet another embodiment, thesemiconductor is deposited first, prior to depositing the source anddrain electrodes. In most embodiments, a field effect transistorincludes an insulating layer, a gate electrode, a semiconductor layerincluding an inorganic material as described herein, a source electrode,and a drain electrode, wherein the insulating layer, the gate electrode,the semiconductor layer, the source electrode, and the drain electrodeare in any sequence as long as the gate electrode and the semiconductorlayer contact opposite sides of the insulating layer and the sourceelectrode and the drain electrode both contact the semiconductor layer.

For conductive layers, such as those used for the gate, source, anddrain electrodes, useful materials include metals and conductive metaloxides. Examples of useful conductors include Al, Cr, Mo, Ag, aluminumdoped ZnO (AZO), and indium doped tin oxide (ITO). The gate dielectriccan be an inorganic gate dielectric, for example an insulating metaloxide such as aluminum oxide, HfO, ZrO, or SiO2. The inorganicsemiconductor layer can be a metal oxide semiconductor, for example aZnO based semiconductor. These layers of the TFT can be deposited usingany method known in the art, including CVD, PECVD, sputtering orevaporation. ALD is a useful method of deposition, and more specificallyspatial ALD due to the low required processing temperature. The activelayers can be patterned using methods including, selective deposition,sequential masking, photolithography, laser, and/or other means known tothe skilled artisan.

Atomic Layer Deposition (ALD) is a process which is used to producecoatings with thicknesses that can be considered consistent, uniform, oreven exact. ALD produces coatings that can be considered conformal oreven highly conformal material layers. Generally described, an ALDprocess accomplishes substrate coating by exposing the substrate toalternate cycles of two or more reactive materials, commonly referred toas precursors. Traditional ALD systems are chamber based, and operateunder vacuum. In a typical process, a first precursor is applied toreact with the substrate, followed by the removal of the excess of thefirst precursor from the vacuum chamber. Next, a second precursor isapplied to react with the first precursor on the substrate, followed bythe removal of the excess of the second precursor from the vacuumchamber. This process is repeated to build up the coating thickness witheach reaction happening only at the substrate surface.

In a preferred embodiment, ALD can be performed at or near atmosphericpressure and over a broad range of temperatures, preferably at atemperature of under 300° C. Optionally, the present process can beaccomplished using a new ALD process which negates the need for a vacuumchamber. This process, commonly referred to as S-ALD, is described in atleast one of commonly assigned U.S. Pat. Nos. 7,413,982; 7,456,429;7,789,961; and US Patent Application Publication No. US 2009/0130858.All of the above-identified patents and patent applications areincorporated by reference herein in their entirety.

S-ALD produces coatings with thicknesses that can be consideredconsistent, uniform, or even exact. S-ALD produces coatings that can beconsidered conformal or even highly conformal material layers. S-ALD isalso compatible with a low temperature coating environment.Additionally, S-ALD is compatible with web coating, making it attractivefor large-scale production operations. Even though some web coatingoperations may experience alignment issues, for example, web tracking orstretching issues, the vertical transistor architectures of someembodiments of the present invention can reduce reliance on highresolution or very fine alignment features during the manufacturingprocess. Therefore, S-ALD is well suited for manufacturing the presentinvention.

The preferred process of the present invention employs a continuousspatially dependent (as opposed to pulsed or time-dependent ALD) gaseousmaterial distribution. The process of the present invention allowsoperation at atmospheric or near-atmospheric pressures and is capable ofoperating in an unsealed or open-air environment. The process of thepresent invention is adapted so that material is deposited only inselected areas of a substrate.

Metal oxides are particularly suited for deposition using ALD,particularly S-ALD. These include, but are not limited to: zinc oxide(ZnO), aluminum oxide (often called Al₂O₃, though stoichiometry is notnecessarily exact), hafnium oxide, zirconium oxide, indium oxide, tinoxide, and the like. Mixed structure oxides that can be made using theprocess of the present invention can include, for example, InZnO. Dopedmaterials that can be made using the process of the present inventioncan include, for example, ZnO:Al, Mg_(x)Zn_(1-x)O, and LiZnO.

Returning now to the discussion of the transistor, in embodiments of thenovel TFTs of the present invention, the back channel of the inorganicsemiconductor (the side opposite the gate) is in contact with a polymerdielectric layer. For bottom-gate devices, that means there is amultilayer insulating structure of the current invention in contact withthe semiconductor. For top-gate devices, there is a polymer dielectricunder, and in contact with, the inorganic semiconductor layer forcharge-control purposes. The specific examples of transistorconfigurations described herein are for illustrative purposes and shouldnot be considered as limiting the scope of the invention.

Embodiments of the present invention relate to polymer insulating layersuseful in controlling the back channel of inorganic semiconductor thinfilm transistors and to multilayer insulating structures containingpolymer insulating layers and thin film inorganic insulating layers.Additionally, a novel method for forming a useful multilayer insulatingstructure is disclosed that uses selective area deposition (SAD) incombination with atomic layer deposition (ALD). As discussed above, theinorganic thin film transistors can be formed by any process known inthe art. Preferred methods of fabricating the transistors include usingSAD in combination with spatial ALD.

The phrase “polymer dielectric” as used herein refers to a polymericmaterial used as an electrically insulating layer and that is present inthe final application, and is useful to distinguish the polymerdielectric material from other polymeric materials or polymer layersthat may be used in the process. The polymer insulating layer of themultilayer insulating structure is composed of a polymer dielectric.Similarly, the polymer structure of the multilayer dielectric structuresis composed of one or more layers of polymer insulating layers. Thepolymer dielectric of the present invention is preferably a polymer thatcan be solution deposited, and is mechanically stable in the finalapplication. Illustrative of polymer dielectrics are polyimides, phenoxyresins, acrylic resin, epoxy resins, polyacrylates, polyvinyl phenol,polyvinyl pyrrolidone, organosiloxanes (such as PDMS) and the like ortheir combinations and blends. The preferred polymer dielectrics areepoxy resins and polyimides. The polymer can be curable, with eitherthermal or radiation curable composition. The polymer does not need tobe radiation curable or photosensitive, but photosensitive formulationsare useful in the present invention so long as the final cured polymerlayer has the structural and mechanical properties required in the finalapplication. Polyimide is a preferred structural polymer dielectric dueto the combination of film properties such as low stress, low CTE, lowmoisture uptake, high modulus and good ductility for microelectronicapplications. Epoxy resins are also preferred due to their thermal andchemical properties. Radiation curable compositions comprising a highlybranched, multifunctional epoxy bisphenol A-novolac resins, such as EponSU-8 from Momentive Specialty Chemicals Inc. is one example of a usefulepoxy resin.

Functionalized poly(siloxanes) include epoxy-functionalized,carboxyl-functionalized, polyether-functionalized,phenol-functionalized, amino-functionalized, alkoxy-functionalized,methacryl-functionalized, carbinol-functionalized,hydroxy-functionalized, vinyl-functionalized, acrylic-functionalized,silane-functionalized, trifluoro-functionalized, ormercapto-functionalized poly(organosiloxanes). Block copolymers can alsobe employed if containing substantial siloxane repeat units. Suchpolymers can be prepared as described in numerous patents andpublications or are commercially available from, for example, GeneralElectric Company, Schenectady, N.Y.; Dow Corning, Midland, Mich.; orPetrarch Systems, Bristol, Pa.

The polymer structure of the multilayer insulating structure ispatterned. The patterning of the polymer structure is usefullyaccomplished using the patterned inorganic thin film insulating layer asa hard mask to etch a blanket-deposited polymer dielectric.Alternatively a “positive-acting” polymer (the polymer is removed whereit has been exposed) can be used as the polymer dielectric, and thepatterned inorganic thin film insulating layer can be used as aphoto-mask for exposing the dielectric layer. In both cases, theresultant polymer insulating layer has a pattern that is defined by thepattern of the inorganic thin film layer. The pattern of the inorganicthin film layer defines a first area, the pattern of the polymerstructure defines a second area, and the second area is located with thefirst area. The second area of the polymer structure does not extendbeyond the first area of the inorganic dielectric layer. In someembodiments, the edges of the patterned polymer structure will bealigned with the edges of the patterned inorganic thin film insulatinglayer. In these embodiments, the first area and the second area are thesame. In other embodiments, the edges of the patterned polymer structurewill lie within the pattern of the edges of the patterned inorganic thinfilm insulating layer due to over etching or over exposure of thepolymer dielectric. In these embodiments, the second area is smallerthan the first area and is located within the first area. The polymerstructure can be composed of one or more polymer layers; the pattern ofthe polymer structure is defined by the pattern of the polymer layerhaving the largest area.

In yet further embodiments, a second inorganic thin film layer can beconformally coated over the polymer insulating layer and the patternedinorganic layer, thus sealing the edges of both. In these embodiments itis preferred that the second inorganic thin film layer is alsopatterned. The pattern of the second inorganic thin film layer is suchthat the thin film layer seals the edges of the polymer and inorganiclayer, but has openings allowing contact to, for instance, theelectrodes of the TFT. The second inorganic thin film has a thirdpattern defining a third area that is larger than both the first andsecond areas. The patterned inorganic thin film dielectric can be formedin using any methods known in the art. Preferably, inorganic thin filmdielectric is deposited using ALD, more preferably using spatial ALD.The inorganic thin film layer can be deposited uniformly and patternedusing any method known in the art, including using a photolithographicprocess. Preferably, the inorganic thin film layer can be patterned asdeposited by using the combination of selective area deposition (SAD)and ALD.

SAD employs a patterned material referred to as a “deposition inhibitormaterial”, “deposition inhibiting material”, or simply an “inhibitor”that inhibits the growth of a thin film material on the substrate whenthe substrate is subjected to an atomic layer deposition. By inhibitingthe growth where the deposition inhibiting material is present, thedeposition only occurs in regions (selective areas) of the substratewhere the inhibitor is not present. The phrase “deposition inhibitormaterial” and its equivalents refer herein to any material on thesubstrate that inhibits the deposition of a film during atomic layerdeposition (ALD). The “deposition inhibitor material” includes thematerial applied to the substrate as well as the material resulting fromany optionally subsequent cross-linking or other reaction that modifiesthe material that may occur prior to depositing an inorganic thin filmon the substrate by atomic layer deposition. A polymeric depositioninhibitor material may be cross-linked after applying the polymer ontothe substrate, before or during the pattering step.

The deposition inhibitor material can be a compound or polymer that,after being applied, is subsequently polymerized, cross-linked, orpolymerized and cross-linked. Examples of polymers include apoly(perfluoroalkyl methacrylate); poly(perfluoroalkyl methacrylate);poly(methyl methacrylate); poly(cyclohexyl methacrylate); poly(benzylmethacrylate); poly(iso-butylene); poly(9,9-dioctylfluorenyl-2,7-diyl);poly(hexafluorobutyl methacrylate), and copolymers thereof, wherein thealkyl has one to six carbon atoms.

Cross-linking can be used to insolubilize a polymeric depositioninhibitor material after application onto the surface of the substrate.The crosslinking can occur prior to patterning or may occur duringpatterning in order to contribute to the patterning step, for example,by employing crosslinking initiated by, and patterned by, actinicradiation, followed by removal of non-crosslinked polymer, for example,by solvent.

The polymeric inhibitor may be soluble in any convenient solvent and mayhave any useful molecular weight, preferably in the range of 2,000 to2,000,000. It may include a single functional group, or may include aplurality of functional groups. In the case of a plurality, the polymermay be a random, periodic, or block polymer. For polymers with chiralcenters the polymer may be isotactic, syndiotactic, or atactic. Thepolymer may have side chains and may be a graft copolymer. The polymermay be linear or branched. The polymer may have low numbers of free acidgroups. Preferred polymers that are soluble in non-polar solvents arepoly(methylmethcrylate) or epoxy resins. Polymers soluble in polarsolvents such as water, alcohols, or ketones are particularly useful.Polymers may include amide groups, such as poly(amide),poly(vinylpyrollidone), and poly(2-ethyl-oxazoline. A particularlyuseful deposition inhibitor is poly(vinylpyrollidone).

The deposition of the deposition inhibitor material can be in apatterned manner, such as using inkjet, flexography, gravure printing,microcontact printing, offset lithography, patch coating, screenprinting, or transfer from a donor sheet. In alternative embodiments, auniform layer of the deposition inhibitor material can be deposited andthen patterned to form a patterned layer of the deposition inhibitormaterial. Pre-processing treatments for patterning the inhibitor includepatterning of substrate prior to inhibitor application to modify thehydrophobilicity, electric charge, absorption, or roughness of thesubstrate. Post-processing treatments include light exposure, lightexposure and subsequent liquid based development, plasma etching, andablation.

Providing the patterned deposition inhibiting material layer on thesubstrate can include using at least one of an inkjet printing process,a flexographic printing process, a gravure printing process, and aphotolithographic process. The active inhibiting material may besuspended or dissolved in a solvent or vehicle. The material may includesurfactants, stabilizers, or viscosity modifiers. The printed materialmay be dried using natural convection, forced convection, or radiantheat. The material may be treated to change its morphology or chemicalcomposition. A preferred chemical composition change is to crosslink thematerial. The change in morphology or chemical composition may beaccomplished by exposure to a vapor phase or liquid phase reactant, ortreatment with heat or light. Preferred processes include thecrosslinking of material with UV light.

In some applications it is desirable to have a polymer dielectric havingthe same pattern as the polymer inhibitor used to pattern an ALDdeposited inorganic thin film layer. In these applications, the polymerinhibitor is selected to advantageously have inhibitor properties anddielectric properties, as well as mechanical properties such that thepolymer is stable in the final structure and application. When thepolymer inhibitor is coated with another polymer dielectric layer it maynot be necessary to modify the polymer inhibitor surface. In someembodiments, however, the polymer inhibitor is a switchable polymerinhibitor. A switchable polymer inhibitor is a polymer that asdeposited, or as patterned, inhibits ALD growth of inorganic thin films.Upon subjecting the switchable polymer inhibitor to a suitabletreatment, the polymer loses its inhibition properties and the ALD isnow able to nucleate on the polymer surface. Suitable treatments includehigh energy oxygen processes, such as oxygen plasmas and UV-ozonetreatments. Preferred switchable polymer inhibitors include epoxy resins(such as SU-8), organosiloxanes (such as PDMS), and polyvinylpyrrolidone.

A process of making the multilayer insulating structure of the presentinvention can be carried out below a support temperature of about 300°C., more preferably below 250° C., or even at or around 100° C. Thesetemperatures are well below traditional integrated circuit andsemiconductor processing temperatures and enable the use of any of avariety of relatively inexpensive supports, such as flexible polymericsupports. Thus, an embodiment of the invention enables production ofrelatively inexpensive devices on flexible substrates without the needfor photolithography and enabling rapid pattern changes due to printingthe patterns. An embodiment of the invention also enables production ofrelatively inexpensive circuits containing thin film transistors withsignificantly improved performance due to the control of the backchannel interface and the presence of a passivation layer. Additionally,the low processing temperature required for the multilayer insulatingstructure reduce the potential for undesirable changes in deviceperformance due to thermal cycling.

The substrates used in the present invention can be any material thatacts as a mechanical support for the subsequently coated layers. Thesubstrate can include a rigid material such as glass, silicon, ormetals. Useful substrate materials include organic or inorganicmaterials. Flexible supports or substrates can be used in the presentinvention. Nominally rigid materials that are flexible due to theirthinness may also be used. These include glass at thicknesses below 200μm and metals at thicknesses below 500 μm.

The substrate can be bare indicating that it contains no substantialmaterials on its surface other the material from which it is composed.The substrate can include various layers on the surface. These layersinclude subbing layers, adhesion layers, release layers, wetting layers,hydrophilic layers, and hydrophobic layers. The substrate surface can betreated in order to promote various properties. These treatments includeplasma treatments, corona discharge treatments, and chemical treatments.

The substrate can also include on its surface patterned materials. Thesepatterns may include patterns that modulate light transmission orelectrical conductivity within or on the substrate. The patterns mayinclude complete devices, circuits, or active elements existing on thesubstrate. The patterns may include portions of devices, circuits, oractive elements awaiting subsequent processing steps for completion.

The following descriptions are provided to set forth features of thepresent invention and serve to illustrate the key aspects of themultilayer insulating structure. The figures provided are not drawn toscale but are intended to show overall function and the structuralarrangement of some embodiments of the present invention. Additionally,the invention is not limited to the embodiments shown and it should beunderstood that variations and modifications can be effected within thescope of the invention

For bottom gate thin film transistors, it is necessary to both controlthe back channel interface and protect the device from environmentalconditions by passivating the device. The multilayer insulatingstructure of the present invention as utilized in a bottom gate TFT canbe better understood with respect to FIGS. 1 a and 1 b. FIG. 1 a is across-sectional diagram of one embodiment of a TFT 101 of the presentinvention, taken along the line A-A′ of the plan view shown in FIG. 1 b.The TFT 101 shown in FIGS. 1 a and 1 b is a bottom gate structure thatis representative of any bottom gate TFT 101 where the gate 120 is incontact with the substrate 110, the dielectric insulating layer 130 isin contact with the gate 120 and the substrate 110, and the inorganicsemiconductor layer 150 is in contact with the source/drain 160/165. Thesubstrate 110 can be any previously discussed substrate, and may containa plurality of predefined layers. The gate 120 has the conventionallyaccepted meaning, and is used to modulate the current of the TFT 101.The electrically conductive gate layer of the gate 120 may be a singleconductive material, as shown in FIG. 1 a, or may include any number ofconductive material layers. In useful embodiments, the inorganicsemiconductor layer 150 of bottom gate transistor 101 is a metal oxidesemiconductor. In useful embodiments, the metal oxide semiconductor is aZnO based semiconductor.

In order to keep the gate 120 isolated from the semiconductor 150, thesource electrode 160, and the drain electrode 165, an insulating layer130 is disposed between the gate 120 and the semiconductor 150, thesource electrode 160, and the drain electrode 165. Preferably,insulating layer 130 is a patterned inorganic thin film dielectriclayer. Insulating layer 130 can be a single dielectric material, asshown in FIG. 1 a, or may include any number of dielectric materiallayers.

The interface between the semiconductor 150 and the dielectricinsulating layer 130 is critical to the function of the TFT 101.Depending upon the manufacturing methods used to make the TFT 101, thisinterface may or may not be easily controlled. In TFTs 101 that areformed by the combination of SAD and ALD, special care should be takento insure that when changing between the dielectric pattern and thesemiconductor pattern the interface is not disturbed by the removal ofthe deposition inhibiting material. As shown in FIG. 1 a the inorganicsemiconductor layer 150 is in direct contact with the patternedinorganic thin film dielectric insulating layer 130. In an alternativeembodiment (not shown) TFT 101 can include a patterned third inorganicthin film dielectric layer, also referred to as a buffer layer, locatedbetween and in contact with the inorganic thin film dielectric layer 130and the semiconductor layer 150. Preferably, the patterned inorganicsemiconductor layer 150 has the same pattern as the buffer layer.

FIGS. 1 a and 1 b show the relative location of the source electrode 160and the drain electrode 165 on substrate 110. The source and drain haveconventionally accepted meanings, and the electrodes are noted as sourceand drain for clarity of understanding—either electrode shown mayfunction as the source (or drain) as is required by the application orcircuit. The source electrode 160 and drain electrode 165 are formed inan electrically conductive layer. The electrically conductive layer canbe a single conductive material, as shown in FIG. 1 a, or may compriseany number of conductive material layers.

FIGS. 1 a and 1 b show the multilayer insulating structure 190 whichincludes a polymeric structure 170 in contact with the inorganicsemiconducting layer 150 and an inorganic dielectric layer 180 over thepolymeric structure 170. As shown both in the cross-sectional view ofFIG. 1 a and the plan view of FIG. 1 b, the inorganic dielectric layer180 has the same pattern as polymeric structure 170. The multilayerinsulating structure 190 functions as a passivation layer for the TFT101.

Multilayer insulating structure 190 shown in FIGS. 1 a and 1 b includesa polymer structure 170 that is formed from a single polymer dielectriclayer 171. In such embodiments the polymer structure 170 and the polymerlayer 171 are the same, and therefore have the same pattern. As shown inFIG. 1 a, the multilayer insulating structure 190 is in contact with atleast the inorganic semiconductor layer 150 in the gap between thesource and drain electrodes 160,165. The gap defines the channel of thethin film transistor 101, and the multilayer insulating structure can besaid to passivate the backchannel of the thin film transistor 101.

As shown, the inorganic dielectric layer 180 has a first patterndefining a first area; and the polymer structure 170 has a secondpattern defining a second area. The second area is located within thefirst area, and the polymer structure 170 is in contact with thesemiconductor layer 150 in the gap. Thin film transistor 101, having themultilayer dielectric structure 190 of the present invention ispreferably an enhancement mode device.

There are a number of embodiments of the multilayer insulating structure190 useful in the present invention that have the common elements of apolymer structure and an inorganic dielectric layer as illustrated inFIGS. 2 a through 2 p. In all examples, there is a patterned polymerstructure 170 and a patterned inorganic thin film dielectric layer 180.The patterned polymer structure 170 has a pattern defining an area thatis confined within the limits of the area of the patterned inorganicthin film dielectric layer 180, such that the edges of the patternedpolymer structure 170 are aligned with or lie within the area of thepatterned inorganic thin film layer 180. FIG. 2 a is a cross-sectionalview taken along the line A-A′ of FIG. 2 b. FIGS. 2 a and 2 b illustratean embodiment of the multilayer insulating structure 190 that is thesame as that shown in FIGS. 1 a and 1 b. Although shown in FIG. 2 awithout topology, it can be appreciated that the polymer structure 170and inorganic thin film 180 can be used to cover features having anassociated height, and therefore can have height variations (asillustrate in FIG. 1 a). As shown, the first area (the area of thepattern of the polymer structure 170) is equal to the second area (thearea of the pattern of the inorganic thin film dielectric layer 180).

FIG. 2 c is a cross-sectional view taken along the line A-A′ of FIG. 2d. FIGS. 2 c and 2 d illustrate an embodiment of the multilayerinsulating structure 190 wherein the polymer structure 170 has at leasttwo layers; a first patterned polymeric layer 171 having the secondpattern and a second polymeric layer 175 has a third pattern thatdefines a third area. The third area is located with the second areadefined by the second pattern layer, and the pattern of the polymerstructure 170 can be said to be that of the first patterned polymerlayer 171 (i.e. the second pattern). As shown in FIG. 2 d, the patternof the first polymer layer 171′ is aligned with the pattern of theinorganic thin film dielectric layer 180. The pattern of the thirdpolymer layer 175′ is within the boundaries of the first patternedpolymer layer 171′. FIG. 2 e is a cross-sectional view taken along theline A-A′ of FIG. 2 f. FIGS. 2 e and 2 f are similar to that describedin relationship to FIGS. 2 a and 2 b, where the multilayer insulatingstructure 190, includes a patterned polymer structure 170, and theinorganic thin film dielectric layer. These should be understood fromthe previous description. In FIGS. 2 e and 2 f, the first patternedpolymer layer 171 has been etched back so that the first area (the areaof the pattern of the inorganic thin film layer 180) extends beyond thesecond area (the area of the pattern 170′ of the polymer structure 170)to define a reentrant profile between the inorganic dielectric layer 180and the patterned polymeric structure 170.

FIG. 2 g is a cross-sectional view taken along the line A-A′ of FIG. 2h. FIGS. 2 g and 2 h are similar to FIGS. 2 c and 2 d, where themultilayer insulating structure 190 has a polymer structure 170 with atleast two layers, and wherein the first patterned polymeric layer 171has the second pattern and the second polymeric layer 175 has a thirdpattern that defines a third area. In FIGS. 2 g and 2 h, the firstpatterned polymer layer 171 has been etched back so that the first area(the area of the pattern of the inorganic thin film layer) extendsbeyond the second area (the area of the pattern 170′ of the polymerstructure 170) to define a reentrant profile between the inorganicdielectric layer 180 and the polymeric structure 170. FIGS. 2 i through2 p are analogous to FIGS. 2 a through 2 h, with the added feature ofanother inorganic dielectric layer 185 that is conformal and extendsbeyond the first area. The other features of the multilayer dielectricstructure 190 should be understood from the previous descriptions ofFIGS. 2 a through 2 h.

FIG. 3 is a cross-sectional diagram of another embodiment of a bottomgate TFT 103 of the present invention. The plan view shown in FIG. 1 b,is equivalent to the plan view for the TFT 103 of FIG. 3. As shown inFIG. 3, the multilayer insulating structure 190 includes a polymerstructure 170 having two polymer layers. The multilayer insulatingstructure 190 shown in FIG. 3 is equivalent to that shown in FIGS. 2 cand 2 d. As shown in FIG. 3, the second polymer layer 175 is in contactwith the semiconductor layer 150, and in contact with the firstpatterned polymer layer 171. The second polymer layer 175 preferably hasthe same pattern as the gap between the source and drain, and in someembodiments the second polymer layer 175 is used to pattern the sourceand drain electrodes 160,165 using an ALD and SAD process. In theseembodiments, the second polymer layer 175 is both a polymer dielectricand a deposition inhibitor material. In one preferred embodiment, thepolymer layer 175 is polyvinylpyrrolidone. Multilayer insulating layer190 also contains the inorganic thin film layer 180 over the patternedpolymer structure 170. As shown, the first polymer layer 171 is incontact with both the second polymer layer 175 and the inorganic thinfilm layer 180. The patterned polymer structure 170 is not limited totwo polymer layers. The patterned polymer structure 170 and theinorganic thin film layer 180 can have the same pattern, or thepatterned polymer structure 170 can have a pattern such that each layerof patterned polymer structure 170 is located completely within the areaof the pattern of the inorganic thin film layer 180.

FIG. 4 is a cross-sectional diagram of another embodiment of a bottomgate TFT 104 of the present invention. The plan view shown in FIG. 1 b,is equivalent to the plan view for the TFT 104 of FIG. 4. As shown inFIG. 4, the multilayer insulating structure 194 includes a polymerstructure 470 that is in contact with the semiconductor layer 150. Thepolymer structure 470 preferably has the same pattern as the gap betweenthe source and drain electrodes 160, 165, and in some embodiments thepolymer layer 470 was used to pattern the source and drain electrodes160,165 using an ALD and SAD process. In these embodiments, the polymerstructure 470 is formed from a switchable polymer inhibitor which isboth an insulating polymer and a deposition inhibitor material. Asdescribed above, a switchable polymer inhibitor can lose its inhibitingproperty after exposure, for example, to an oxygen plasma or UV-ozoneprocess. In one preferred embodiment, the polymer structure 470 ispolyvinylpyrrolidone; in other embodiments a preferred polymer for thepolymer structure 470 is a novalac resin. Multilayer insulatingstructure 194 also contains inorganic thin film layer 180 that is overthe polymer structure 470. Preferably, the inorganic thin film layer 180is a conformal inorganic dielectric layer, preferably patterned using acombination of SALD and SAD. Using a switchable inhibitor polymer topattern the source and drain electrodes 160,165 using an ALD and SADprocess allows for a simpler fabrication of the passivated transistor104. There is no need to remove the inhibitor or add an additionalpolymer layer.

FIG. 5 a is a cross-sectional diagram of one embodiment of a TFT 105 ofthe present invention, taken along the line A-A′ of the plan view shownin FIG. 5 b. TFT 105 is a bottom gate structure (as in FIG. 1 a) wherethe gate 120 is in contact with the substrate, the insulating layer 130is in contact with the gate 120 and the substrate 110, and thesemiconductor layer 150 is in contact with the source and drainelectrodes 160,165. These layers should be understood from theirdescriptions with respect to FIGS. 1 a and 1 b.

FIGS. 5 a and 5 b show the multilayer insulating structure 190 whichincludes the patterned polymer structure 170 in contact with thesemiconducting layer 150, the first inorganic dielectric layer 180 incontact with the polymeric layer 171, and the second inorganicdielectric layer 185 in contact with the first inorganic dielectriclayer 180. The structure is analogous to the structures illustrated inFIGS. 2 i and 2 j. In alternative embodiments, the structures shown inFIGS. 2 m and 2 n could be used, where there is a reentrant profilebetween the patterned polymer structure 170 and the inorganic thin filmdielectric layer 180. As shown, the patterned polymer structure 170 andthe inorganic thin film layer 180 have the same pattern. The secondinorganic dielectric layer 185 extends beyond the edges of the firstinorganic dielectric layer 180 and of the polymer structure 170. Thesecond inorganic dielectric layer 185 can be a different material thanthat of the first inorganic thin film dielectric layer 180. The secondinorganic thin film dielectric layer 185 is preferably the same materialas the first inorganic thin film dielectric layer 180; in theseembodiments it may be difficult to detect the boundary between layers180 and 185. The structure will instead be recognized due to the greaterthickness of inorganic dielectric material located over the polymerstructure 170, compared to the thickness of inorganic dielectricmaterial not located over the polymer structure 170. The secondinorganic dielectric layer 185 is preferably a conformal layer, as onewould obtain from deposition using an ALD process. The second inorganicthin film dielectric layer 185 seals the edges of the polymer structure170.

FIG. 5 c is a cross-sectional diagram of a bottom gate TFT 106 of thepresent invention having an alternative multilayer insulating structure,taken along the line A-A′ of the plan view shown in FIG. 5 b. Theelements in TFT 106 are the same as those in FIG. 5 a, however themultilayer insulating structure 190, has two polymer layers 171,175 thatmake up the patterned polymer structure 170. The structure themultilayer insulating structure 190 of FIG. 5 c is analogous to thestructures illustrated in FIGS. 2 k and 2 l. In alternative embodiments,the structures shown in FIGS. 2 o and 2 p could be used, where there isa reentrant profile between the patterned polymer structure 170 and theinorganic thin film dielectric layer 180. Second polymer layer 175 ispreferably a polymer that is both a deposition inhibitor and aninsulating material. Second polymer layer 175 should be understood fromthe description of second polymer layer 175 in relationship to FIG. 3,and preferably has the same pattern as the gap between the source anddrain electrodes 160, 165. Similarly, in some embodiments the secondpolymer layer 175 was used to pattern the source and drain electrodes160,165 using an ALD and SAD process. In these embodiments, the secondpolymer layer 175 is both polymer dielectric and a deposition inhibitormaterial. In one preferred embodiment, the second polymer layer 175 ispolyvinylpyrrolidone.

In some applications it is desirable to fully encapsulate one or moredevices. FIGS. 6 a and 6 b illustrate embodiments in which themultilayer insulating structure 195 covers the entire area of the thinfilm transistor 116, and the source, drain and gate electrodes 160, 165,120 have electrical contacts running elsewhere at the level of thesubstrate 110. In alternative embodiments, the polymer structure 170 andfirst inorganic thin film layer 180 can cover the entire area of thethin film transistor 116, however the pattern of the polymer structure170 will still be defined by the first inorganic thin film layer 180,and in the cases where there is a second inorganic thin film layer 185it will serve to seal the edges of the polymer layer. In still otherembodiments, multiple devices can be covered with a contiguous patternarea of the multilayer insulating structure 195.

The multilayer insulating structure can be a passivating layer asdescribed in the previous examples or it can be used as a second gatedielectric layer. FIG. 7 a is a cross-sectional diagram of anotherembodiment of a TFT 107 of the present invention, taken along the lineA-A′ of the plan view shown in FIG. 7 b. As shown in FIG. 7 a, TFT 107is a dual gate structure where the semiconductor channel can be gatedfrom the top gate, the bottom gate or both. As shown, the bottom gate120 is in contact with the substrate 110, the insulating layer 130 is incontact with the gate 120 and the substrate 110, and the inorganicsemiconductor layer 150 is in contact with the source and drainelectrodes 160,165. These layers should be understood from theirdescriptions with respect to FIGS. 1 a and 1 b.

FIGS. 7 a and 7 b show the multilayer insulating structure 190 whichincludes the patterned polymer structure 170 in contact with theinorganic semiconducting layer 150 and an inorganic dielectric layer 180in contact with the patterned polymeric structure 170. These layersshould be understood from the previous discussion. In a dual gate TFT107, the multilayer insulating layer 190 also functions as thedielectric layer for the top gate 125. The top gate 125 has theconventionally accepted meaning, and is used to modulate the current ofthe TFT 107. The top gate 125 can be a single conductive material, asshown in FIG. 7 a, or may comprise any number of conductive materiallayers. Any of the previously discussed multilayer insulating structures190 can be used in the dual-gate geometry of FIGS. 7 a and 7 b.

Some embodiments of the present invention relate to short channelvertical thin film transistors (VTFTs) which contain a gate structurethat defines a vertical portion of the channel. The term verticaltransistor as used herein refers to transistors where the source anddrain which define a single channel are at two different distances fromthe substrate surface (as measured orthogonal to the substrate). Thisarrangement results in vertical transistors where at least a portion ofthe channel is vertically oriented with respect to the substrate, whichis to say, not parallel to the top surface of the substrate. Verticaltransistors of the present invention include those that have portions oftheir channels in an arrangement that is parallel to the substratesurface, as long as they have a portion which is not parallel. Thevertical transistors of the present invention are similar to bottom gatetransistors, in that they have an exposed back channel as fabricated andit is therefore desirable to both control the back channel interface andprotect the device from environmental conditions by passivating thedevice.

A vertical transistor embodiment of the present invention includes asubstrate, and an electrically conductive gate structure having areentrant profile on the substrate. There is a conformal gate insulatinglayer in contact with the gate structure in the reentrant profile, and aconformal semiconductor layer in contact with the conformal gateinsulating layer. The source and drain electrodes of the verticaltransistor are a first electrode in contact with a first portion of theconformal semiconductor layer over the electrically conductive gatestructure and a second electrode that is in contact with a secondportion of the conformal semiconductor layer and separated verticallyfrom the first electrode. The backchannel of the transistor is protectedby the multilayer insulating structure which in contact with at leastthe conformal semiconductor layer in the reentrant profile. Themultilayer insulating structure includes an inorganic dielectric layerand a polymer structure that is in contact with the conformalsemiconductor layer in the reentrant profile. The multilayer insulatingstructure of the present invention as utilized in a vertical TFT can bebetter understood with respect to the following description.

Embodiments of the vertical transistors of the present invention areformed using a gate structure. As used herein, the gate structure can beany structure which has a reentrant profile. The gate structure can beuniform in material composition, or be formed of multiple materials. Thegate structure can also be referred to as a post, as long as the posthas the required reentrant profile. The gate structure can be formedusing a post and cap, where the cap extends beyond the wall of the postto define the reentrant profile. The gate structure can be formed fromany material, including both conductive and insulating materials. Theelectrically conductive gate structure, as used herein, refers to thestructure which contains the conductive material which will be used togate the vertical transistor channel. The electrically conductive gatestructure is at least conductive on the surface within the reentrantprofile. In some embodiments, the electrically conductive gate structureincludes a gate structure with an electrically conductive gate layer.

Returning to the Figures, a schematic cross-sectional view of verticaltransistors 108 and 208 of the present invention is shown FIG. 8 a,taken along the line A-A′ of the plan view shown in FIG. 8 b.

As shown in FIG. 8 a, TFT 108 and TFT 208 each have a verticaltransistor structure, where the vertical portion is defined by the firstreentrant profile 840 and second reentrant profile 845 of theelectrically conductive gate structure 820 respectively. The insulatinglayer 850 is in contact with the electrically conductive gate structure820 and the substrate 810, and the semiconductor layer 860 is in contactwith the insulating layer 850 and the first electrode 870, the secondelectrode 875, and the third electrode 880. As shown, the firstelectrode 870 and the second electrode 875 are located adjacent to thefirst and second reentrant profiles 840, 845, respectively. The firstelectrode 870 and the second electrode 875 are vertically spaced fromthe third electrode 880 due to the height of the electrically conductivegate structure 820. The third electrode 880 is in conformal contact witha third portion of the semiconductor layer on the top of theelectrically conductive gate structure, the first electrode 870 and thethird electrode 880 defining ends of a first channel of a firsttransistor 800, the second electrode 875 and the third electrode 880defining ends of a second channel of a second transistor 208.

Vertical transistors 108 and 208 are connected in series and formed overa single gate structure. For simplicity, the following description willrelate to vertical transistor 108 with the understanding that thedescriptions apply equally to vertical transistor 208. Verticaltransistor 108 includes a substrate 810. Substrate 810, often referredto as a support, can be rigid or flexible and should be understood fromthe previous descriptions. As shown, the electrically conductive gatestructure 820 is on the substrate, and extends away from the substrateto a top. The electrically conductive gate structure includes a firstedge including a first reentrant profile 840 and a second edge locatedopposite the first edge and including a second reentrant profile 845.

The electrically conductive gate structure 820 can contain bothconductor and insulating materials, the only requirement being that theelectrically conductive gate structure 820 has conductive material onvertical side walls of the reentrant profile 840. The electricallyconductive gate structure 820 can be fully formed of conductivematerials, such as metals and conductive oxides. To better understandthe requirements for the electrically conductive gate structure 820, wewill turn briefly to FIGS. 9 a through 9 c.

As shown in FIGS. 9 a, 9 b and 9 c the electrically conductive gatestructure 820 can have any shape which has a height dimension 730 a, 730b, 730 c extending away from the substrate to a top 735 a, 735 b, 735 c.The electrically conductive gate structure 820 also has edges along theheight dimension, a first edge 710 a, 710 b, 710 c including a firstreentrant profile 840 a, 840 b, 840 c and a second edge 720 a, 720 b,720 c opposite the first edge 710 a, 710 b, 710 c and including a secondreentrant profile 845 a, 845 b, 845 c. The electrically conductive gatestructure 820 has a width at the top 700 a, 700 b, 700 c, and a minimumwidth 705 a, 705 b, 705 c between the top and the substrate that issmaller than the width of the top. The reentrant profile is defined asthe profile of the electrically conductive gate structure 820 that lieswithin the boundaries of the top of the electrically conductive gatestructure 820, the substrate 810, and a line drawn orthogonal to thesubstrate from the edge of the top (as shown by lines P-P′ in FIGS. 9 a,9 b and 9 c). To aid in understanding, the first reentrant profile 840a, 840 b, and 840 c in each figure is shown in bold. Additionally, theelectrically conductive gate structure 820 can have portions that extendbeyond the reentrant profile, as shown in FIG. 9 c. The electricallyconductive gate structure 820 shown in these figures can be fully formedof conductive materials, or in alternative embodiments the electricallyconductive gate structure 820 structures may have an insulating corewith an electrically conductive conformal gate layer coating. FIGS. 9 a,9 b and 9 c illustrate reentrant profiles obtainable with either anelectrically conductive core or insulating structural polymer core. Itshould be recognized that the first and second reentrant profiles shownin FIGS. 9 a, 9 b and 9 c (840 a, 840 b, 840 c and 845 a, 845 b, and 845c) are equivalent to the first and second reentrant profiles 840 and 845of FIG. 8 a.

Returning now to FIGS. 8 a and 8 b, the vertical transistors 108 and 208also include an electrically insulating material layer 850 and aconformal semiconductor material layer 860. The conformal electricallyinsulating layer 850 maintains the reentrant profiles 840 and is incontact with the electrically conductive gate structure 820 and at leasta portion of the substrate 810. The insulating material layer 850conforms to the reentrant profiles 840,845 of the electricallyconductive gate structure 820 in transistors 108 and 208. Insulatingmaterial layer 850 can coat the reentrant profiles 840,845 of theelectrically conductive gate structure 820 and a portion of thesubstrate 810. The conformal electrically insulating layer 850preferably has a uniform thickness. Insulating material layer 850 isoften referred to as a dielectric material layer, and as a gateinsulator. Insulating material layer 850 includes first and secondsurfaces with the first surface being in contact with portions ofsurfaces of electrically conductive gate structure 820 and substrate810. Insulating layer 850 is at least within the reentrant profile 840.

Semiconductor material layer 860 conforms to reentrant profile 840 ofthe electrically conductive gate structure 820. As shown, the conformalsemiconductor layer 860 maintains the first and second reentrantprofiles 840 and is in contact with the conformal electricallyinsulating layer 850. Semiconductor layer 860 includes first and secondsurfaces with the first surface being in contact with the second surfaceof insulating layer 850. The conformal semiconductor layer 860preferably has a uniform thickness. The semiconductor layer 860 is atleast within reentrant profiles 840. Preferably, the conformalsemiconductor layer 860 is an inorganic semiconductor layer, preferablya metal oxide semiconductor.

The first electrode 870, second electrode 875 and the third electrode880 are formed simultaneously in a common step and have the samematerial composition and layer thickness. The first electrode 870 is incontact with a first portion of the semiconductor layer 860 and islocated adjacent to the first reentrant profile 840. The secondelectrode 875 is in contact with a second portion of the semiconductorlayer 860 and located adjacent to the second reentrant profile 845. Thethird electrode 880 is in contact with a third portion of thesemiconductor layer 860 on the top of the electrically conductive gatestructure 820. The first electrode 870 and the third electrode 880define ends of a first channel of a first transistor, and the secondelectrode 875 and the third electrode 880 define ends of a secondchannel of a second transistor.

As shown, the electrically conductive gate structure 820 functions asthe gate for transistors 108 and 208. In some example embodiments oftransistor 108, first electrode 870 functions as the drain of transistor108 and third electrode 880 functions as the source of transistor 108.In other example embodiments of transistor 108, first electrode 870functions as the source and third electrode 880 functions as the drain.The semiconductor device is actuated in the following manner. Aftertransistor 108 is provided, a voltage is applied between the firstelectrode 870 and the third electrode 880. A voltage is also applied tothe electrically conductive gate structure 820 to electrically connectthe electrode 870 and the third electrode 880.

Still referring to FIGS. 8 a and 8 b, vertical transistor 208 is formedat the same time as vertical transistor 108 is formed. Transistor 208can be actuated in the following manner. A voltage is applied betweenthe first electrode 875 and the third electrode 880, which is sharedwith vertical transistor 108. A voltage is applied to the conductivegate structure 820, which is shared with vertical transistor 108, toelectrically connect the electrode 875 and 880.

Alternatively, transistor 108 and transistor 208 can be actuated inseries by applying a voltage between first electrode 870 and secondelectrode 875. A voltage is applied to the conductive gate structure820, which simultaneously electrically connects first electrode 870 tothird electrode 880 and connects third electrode 880 to second electrode875. This can be advantageous for circuit applications because externalelectrical connections do not need to be made to the elevated thirdelectrode 880.

The reentrant profile 840 of transistor 108 allows a dimension of thesemiconductor material channel of the transistor to be associated withthe thickness (or height) of the electrically conductive gate structure820, which functions as the gate, of transistor 108. Advantageously,this architecture of the present invention reduces reliance on highresolution or very fine alignment features during the manufacture oftransistors that include short channels. Furthermore, the separation ofthe first electrode 870 and third electrode 880 is primarily determinedby the reentrant profile 840 in the electrically conductive gatestructure 820.

Referring back to FIGS. 9 a through 9 c, the precise geometry of theelectrically conductive gate structure 820 can vary as long as it meetsthe requirements of having a height dimension extending away from thesubstrate 810 to a top, edges along the height dimension where a firstedge includes a first reentrant profile 840 and a second edge oppositethe first edge includes a second reentrant profile 845. The electricallyconductive gate structure 820 is only required to be conductive alongthe first 840 and second 845 reentrant profiles. However, in someembodiments the electrically conductive gate structure 820 is formedfrom only conductive materials, and is fully conductive. In otherembodiments, the electrically conductive gate structure 820 is formedusing a polymer post, optionally with an inorganic cap, that isconformally coated by an electrically conductive gate layer. It shouldbe clear that all of the structures of FIGS. 9 a through 9 c could beused in place of the electrically conductive gate structure 820 shown inFIGS. 8 a and 8 c, and are to be considered to be embodiments of thepresent invention. As such, the example embodiments of this inventionshould be understood to work with any electrically conductive gatestructure 820 which meets the requirements as defined herein, and is notlimited to the ones illustrated.

The vertical transistor structure illustrated in FIGS. 8 a and 8 b wasused to form two vertical transistors, 108 and 208, connected in series.It should be understood that similar structures can be used to formsingle vertical transistors, or two independently operable transistors,see, for example, co-assigned pending U.S. patent application Ser. Nos.14/198,631; 14/198,623; 14/198,643; 14/198,633; 14/198,682; 14/198,628;14/198,658; 14/198,647, all filed Mar. 6, 2014. The multilayerinsulating structure 890 can be used in any of these alternate verticaltransistor geometries which use a gate structure having a reentrantprofile.

FIGS. 8 a and 8 b show the multilayer insulating layer 890 whichincludes a polymeric structure 178 in contact with the inorganicsemiconducting layer 850 and an inorganic dielectric layer 188 incontact with the polymeric structure 178. As shown, the polymericstructure 178 can have a single layer of insulating polymer 179. Theinorganic dielectric layer 188 preferably has the same pattern aspolymer structure 178. The multilayer insulating layer 890 can have anynumber of polymer layers and inorganic materials layers, as long as itmeets the requirement that a passivating polymer layer 179 of thepolymer structure 178 is in contact with the inorganic semiconductorlayer 850 and there is an inorganic layer 188 in contact with thepolymer structure 178. Preferably, the inorganic layer 188 has the samepattern as the polymer structure 178. As shown in FIG. 8 a, thepolymeric layer 178 preferably fills reentrant profile 840.

The multilayer insulating structure 890 is equivalent to the previouslydescribed multilayer structure 190. Any of the multilayer insulatingstructures described in relationship to FIGS. 2 a though 2 o are usefulin vertical transistor structures. The multilayer insulating structure890 can have any pattern as long as it covers the back channel of thevertical transistor. As shown in FIG. 8 a, the multilayer insulatingstructure 890 can be in contact with a portion of the first electrode870 and a portion of the second electrode 875. As illustrated in FIGS. 2e and 2 f, the multilayer insulating structure 890 can have a polymerlayer with an edge and the inorganic dielectric layer 188 can extendbeyond the edge of the polymeric layer. In some embodiments, theinorganic dielectric layer 188 extends beyond the edge of the polymerlayer to define another reentrant profile between the inorganicdielectric layer 188 and the patterned polymeric structure 170 (alsoshown in FIGS. 2 e and 2 f). FIGS. 10, 11 and 12 illustrate alternativeembodiments of vertical transistors having a multilayer insulatingstructure; these Figures should serve to aid the understanding of theintegration of the multilayer insulating structures of FIGS. 2 a through2 o in vertical transistors.

As shown in FIG. 10, the multilayer insulating layer 890 has a polymerstructure 178 having two polymer layers. The vertical TFT 109 has amultilayer insulating structure 890 with two polymer layers 179, 778 andone inorganic dielectric layer 188. The second polymer layer 778 is incontact with the semiconductor 860 in the channel and in contact withpolymer layer 179. Polymer layer 179 of the polymer structure 178 is incontact with inorganic dielectric layer 188. This is the structure whichwould result from using the polymer 778 as an inhibitor when forming thesource and drain electrodes using an ALD and SAD process, and as suchthe second polymer layer 778 preferably has the same pattern as the gapbetween the source and drain. In these embodiments, the polymer layer778 is both polymer dielectric and a deposition inhibitor material. Inone preferred embodiment, the polymer layer 778 is polyvinylpyrrolidone.

After forming the source and drain electrodes 860, 865, the polymerlayer 179 is then deposited over the inhibitor polymer 778 andoptionally cured. Next a patterned inorganic layer 188 is deposited overthe polymer structure 178, and the patterned inorganic layer 188 is usedas a hard mask to pattern polymer structure 178 using an etchingprocess. This results in a structure, as shown, where the polymerstructure 178 (and polymer layer 179) has the same pattern as thepatterned inorganic layer. This embodiment of the multilayer insulatingstructure 890 is equivalent to that shown in FIGS. 2 c and 2 d.Depending on the etch conditions, the edges of the polymer layer 179 canbe aligned to the edges of the inorganic layer (as shown), or there canbe a reentrant profile where there edges of the polymer layer are insetfrom the edges of the inorganic layer (as illustrated in FIGS. 2 e and 2f).

FIG. 11 illustrates another embodiment of the present invention. Thevertical TFT 111 has a multilayer insulating structure with a singlepolymer layer 179 and two inorganic dielectric layers 188 and 189. Asshown, the vertical TFT 111 is the result of adding an additionalinorganic layer to vertical TFT 108 of FIG. 8 a. The second inorganiclayer 189 covers both the first inorganic layer 188 and the polymerlayer 179 and serves to seal the edges of the polymer layer 179. Theinorganic layer 189 can be a different dielectric material thaninorganic layer 188. Preferably, inorganic layer 189 is the samematerial as inorganic layer 188. When layer 189 and 188 are the samematerial, it may be difficult to detect the boundary between the layers.In this structure, it will be clear because there will be a greaterthickness of inorganic dielectric material over the polymer layer 179,than is found not over the polymer structure 178. The inorganicdielectric layer 189 is preferably a conformal layer, as one wouldobtain from deposition using an ALD process. Vertical transistor 111,has a polymer layer 179 having an edge, with a second conformalinorganic dielectric layer 189 that extends beyond the edge of thepolymer layer 179 and the inorganic dielectric layer 188 to seal theedge of the polymeric layer. The multilayer insulating shown in FIG. 11is equivalent to that shown in FIGS. 2 i and 2 j, where the polymerlayer 179 (polymer structure 178) has the same pattern as the firstinorganic thin film layer 188. Alternatively, the multilayer insulatingstructure 890 illustrated in FIGS. 2 m and 2 n could be used, where thepattern of the first inorganic dielectric layer is larger than that ofthe polymer structure.

FIG. 12 illustrates an embodiment of the present invention. The verticalTFT 112 has a multilayer insulating structure with two polymer layersand two inorganic dielectric layers. As shown, the vertical TFT 112 isthe result of adding an additional inorganic layer 189 to vertical TFT109 of FIG. 10. The structure of FIG. 12 should be understood withrespect to the descriptions of FIGS. 10 and 11. Vertical transistor 112has a polymer structure 178 with first and second polymeric layers 179,778 having edges, with a conformal inorganic dielectric layer 189 thatextends beyond the edges of the first and second polymeric layers 179,778 and the inorganic dielectric layer 188 to seal the edge of thepolymer structure. The multilayer insulating structure 890 shown in FIG.12 is equivalent to the multilayer insulating structure 190 shown inFIGS. 2 k and 2 l. Alternatively, the multilayer insulating structureillustrated in FIGS. 2 o and 2 p could be used, where the pattern of thefirst inorganic dielectric layer 188 is larger than that of the polymerstructure 170. The multilayer insulating structures 890 of the presentinvention preferably cover the majority of the substrate area, and canpreferably be patterned with vias allowing contact to the source, drainand gate electrodes 860, 865, 820. Alternatively, the multilayerinsulating structure 890 having two or more inorganic layers can beformed so that the top-most inorganic layer is unpatterned over a largeregion of the substrate 810, and the electrical connections to the VTFTelectrodes are made by connections running at the substrate level.

Some embodiments of the present invention relate to top-gate thin filmtransistors (TFTs) which, as previously described, have a semiconductorlayer that is between the substrate and the gate layer. Unlike bottomgate TFTs, top gate TFTs are inherently protected from the environmentsince the semiconductor layer is between the substrate and the gatedielectric. The challenge in forming enhancement mode top gate devicesis in controlling the back channel interface, where the semiconductorwould typically contact the substrate. When forming top gate TFTs onsome substrates, including inorganic substrates, the back-channelinterface properties can cause the device to operate in the lessdesirable depletion mode. The present invention utilizes a patternedpolymer layer between the inorganic semiconductor and the substrate tocontrol the interface. In some embodiments the polymer layer can beformed to have the same pattern as the inorganic semiconductor layer, orthe polymer layer can be formed such that the polymer layer is under theinorganic semiconductor layer in the gap and not under the source anddrain electrodes. This method allows the back-channel of the device tohave an interface with a specified material, instead of defaulting to aninterface with the substrate.

FIG. 13 a is a cross-sectional diagram of a top gate TFT 500, takenalong the line A-A′ of the plan view shown in FIG. 13 b. The TFT 500shown in FIGS. 13 a and 13 b is a top-gate structure that isrepresentative of any top gate TFT 500 where a polymer layer 570 is incontact with a substrate 510, the source/drain 560 is in contact withthe polymer layer 570, the semiconductor layer 550 is in contact withthe source/drain 560, the polymer layer 570 and the insulating layer530, and the insulating layer 530 is in contact with the gate 520.

As shown in FIG. 13 b, on substrate 510, there is a polymer layer 570.The substrate 510 can be any previously discussed substrate, and maycontain a plurality of predefined layers. The substrate 510 ispreferably an inorganic substrate. Polymer layer 570 can be anystructural polymer that modifies the charge of the semiconductor backchannel, including SU-8, novalac resin and polyimide based materials.Polymer layer 570 can be a uniform coating over the entire substrate orpatterned. Alternatively, the polymer layer 570 could be self-supportingand serve as both polymer layer 570 and substrate 510. In contact withpolymer layer 570 are source/drain electrodes 560, 565. The source anddrain have conventionally accepted meanings, and either electrode shownmay be designated the source (or drain) as is required by theapplication or circuit. The source/drain 560 can be a single conductivematerial, as shown in FIG. 13 a, or may comprise any number ofconductive material layers. The gap between the source/drain electrodes560/565 defines the channel of TFT 500, as is conventionally understood.The inorganic semiconductor layer 550 is in contact with both thesource/drain 560/565 and the substrate 510, as shown in FIG. 13 a. Theinorganic semiconductor layer 550 can be a metal oxide, for example aZnO-based material.

The insulator layer 530 is in contact with the source/drain electrodes560/565 and the inorganic semiconductor layer 550. Insulating layer 530is preferably patterned to enable electrical contact to the source/drainelectrode 560/565. Insulating layer 530 is often referred to as adielectric material layer, and can be formed of a single dielectricmaterial layer or multiple dielectric material layers. Preferably theinsulating layer 530 is a thin film inorganic dielectric material layer.

Gate layer 520 is over the gap between the source/drain electrodes5601565, and in contact with insulating layer 530. The gate layer 520has the conventionally accepted meaning, and is used to gate the currentof the TFT 500. The gate layer 520 may be a single conductive material,as shown in FIG. 13 a, or may comprise any number of conductive materiallayers.

Although a uniform layer of polymer on the substrate 510 can be usefulto control the charge on the back channel of the semiconductor layer550, it can be difficult to integrate a uniform polymer layer into acircuit design. In many circuits there are conductive traces running atthe level of the substrate 510 that have to connect to the electrodes ofthe thin film transistor 500. This requires that the polymer layer 570that is used to control the charge on the back channel be patterned. Itis preferable that the pattern of the polymer layer 570 be constrictedto be within the pattern of the semiconductor so that it does notnegatively impact any process steps required to integrate the TFTs intovarious circuit designs. Therefore, preferred top-gate transistors ofthe present invention have a substrate and a polymer layer 575 that isin contact with the substrate 510. The polymer layer 575 has a firstpattern defining a first area. Over the polymer layer 575 is aninorganic semiconductor layer 550 that is in contact with the polymerlayer, and has a second pattern defining a second area. The first areaof the polymer layer 575 is located within the second area of theinorganic semiconductor layer 550. A source electrode 560 is in contactwith a first portion of the semiconductor layer 550, and a drainelectrode 565 in contact with a second portion of the semiconductorlayer 550, the source electrode and the drain electrode separated by agap defining the channel of the transistor. There is a gate insulatinglayer 530 in contact with the inorganic semiconductor layer 550 in thegap and a gate in contact with the gate insulating layer 530 over thegap which is used to gate the current of the transistor 500. It ispreferred that the inorganic semiconductor is a metal oxidesemiconductor, and preferably the metal oxide semiconductor is a ZnObased semiconductor. The substrate is preferably an inorganic substratesuch as glass.

FIG. 14 a is a cross-sectional diagram of a top-gate embodiment of a TFT505 of the present invention, taken along the line A-A′ of the plan viewshown in FIG. 14 b. The TFT 505 shown in FIGS. 14 a and 14 b is a topgate structure that is representative of a top gate TFT 505 in which apatterned polymer layer 575 is in contact with the inorganicsemiconductor layer 550 on the side opposite the gate 520, where thepolymer layer 575 has the same pattern as the inorganic semiconductorlayer 550. As shown, the polymer layer 575 has a first pattern whosearea is equal to the second area of the inorganic semiconductor layer530. The area of the polymer pattern can be equal to or less than thatof the pattern of the semiconductor layer 530, and thereby meet therequirement that the first area of the polymer layer 575 is locatedwithin the second area of the inorganic semiconductor layer 550. In someembodiments, the process of patterning the polymer layer 575 may resultin a pattern that is smaller than that of the semiconductor layer 550,due to edge effects from etching or developing. In these embodiments, itis preferred that the amount of over etch is controlled such that thefirst area of the polymer layer 575 is at least 90% of the second areaof the semiconductor layer 550, more preferably such that the first areais at least 95% of the second area.

In the top-gate architecture of TFT 505, the source/drain electrodes560/565 are in contact with the substrate 510 and the semiconductorlayer 550, and the insulating layer 530 is in contact with thesemiconductor layer 550, the source/drain 560, and the gate 520, andthese layers should be well understood from the previous description ofFIGS. 13 a and 13 b. In the top gate TFT 505, at least a portion of eachof the source electrode 560 and the drain electrode 565 are over atleast a portion of the polymer layer 575 and the semiconductor layer550. The polymer layer 575 in FIGS. 14 a and 14 b modifies thesemiconductor back channel interface. In the embodiment shown in FIG. 14a, the polymer layer 575 is patterned in the same pattern as thesemiconductor layer 550. This structure can be obtained by first coatinga uniform polymer layer on the substrate 510. Next, a patternedinorganic semiconductor layer 550 is deposited, preferably using acombination of ALD and SAD processes. The patterned inorganicsemiconductor layer 550 is then used as a hard mask to etch the uniformpolymer layer. The remainder of the device is formed using any processknown in the art. This method of using the patterned inorganicsemiconductor layer 550 to mask the patterning of polymer layer 530allows for precise control of the pattern of the polymer layer 575.

In alternate embodiments, the semiconductor layer 550 can extend beyondthe edges the polymer layer with a pattern that is different from thepattern of the polymer layer, so long as the semiconductor layer 550 isin contact with the polymer layer in the area defined by the gap in thesource/drain electrodes. The top gate TFT 515 shown in FIG. 15 a, across-sectional diagram of along the line A-A′ of the plan view shown inFIG. 15 b, is illustrative of using a switchable polymer inhibitor tocontrol the back channel. The polymer layer 578 is a patternedswitchable inhibiting polymer layer that was used to pattern the sourceand drain electrodes. After patterning the source and drain electrodes560, 565, patterned switchable inhibiting polymer layer 578 was treatedto enable growth on the polymer surface. The inorganic semiconductorlayer 550 is then deposited over the top of the treated insulatingpolymer layer 578. In preferred embodiments, the inorganic semiconductorlayer 550 is patterned at the time of deposition using the combinationof the SAD and ALD processes. The same polymer material as in layer 578can be used as the deposition inhibitor for patterning the semiconductorlayer 550, as long as it is deposited after the step of treating thepolymer layer 578. This use of the patterned inhibitor 578 that was usedto form the source and drain electrodes eliminates a process step whenforming enhancement mode top gate TFTs with a polymer back channelprotection layer. As shown in FIGS. 15 a and 15 b, polymer layer 578 hasa pattern having a first area that is smaller than the second area ofthe semiconductor layer 550. In this embodiment, the first pattern ofthe polymer layer 578 defines the gap that separates the sourceelectrode 560 and the drain electrode 565. Since the polymer layer 578was used to pattern the source and drain electrodes 560,565, the sourceelectrode 560 and the drain electrode 565 are not over the polymer layer578.

The multilayer insulating structures for bottom-gate and verticaltransistors discussed above are preferably formed by the uniformdeposition of a structural polymer, followed by the formation of apatterned inorganic dielectric layer, followed by pattern-wise etchingthe structural polymer. Preferably, this process uses the combination ofSAD and ALD process to form the patterned inorganic dielectric layer asdescribed in co-assigned pending U.S. application Ser. No. 14/198,626,filed Mar. 6, 2014. In some embodiments, the polymer that is in contactwith the back channel of the semiconductor layer was also used as adeposition inhibitor. The ability to switch the polymer propertiesbetween inhibiting and not inhibiting enables the formation ofstructures with polymers as a permanent element. The use of polymerinhibitors materials that can be switched removes process steps indevice fabrication. Where desirable, the polymer can be left in placeand an inhibitor removal step is no longer required. In someembodiments, the step of depositing and patterning a passivating polymercan be removed since the switchable deposition inhibitor can perform asthe desired passivation layer. One embodiment of a process of thepresent invention is outline in FIG. 16 in a diagrammatic Step diagramfor making a patterned polymer layer using a combination of selectedarea deposition (SAD) and ALD where the polymer inhibitor remains in thefinal structure.

As shown in FIG. 16, in Step 10 a substrate is provided. The substratemay be any substrate as discussed that is suitable for use in the ALDsystem, and can contain any number of preexisting features and materiallayers. In Step 15 a patterned polymeric inhibitor layer is provided onthe substrate. The polymeric inhibitor material can be any polymermaterial that both causes the ALD material deposition to be inhibitedand is desired to remain in the final structure. Useful polymers includethose that are stable under device conditions. Epoxy resins, novalacresins, and polyvinyl pyrrolidone are preferred polymer inhibitormaterials. The patterned polymeric inhibitor layer is preferably printedin a patterned manner, using a printing means such as using inkjet,flexography, gravure printing, microcontact printing, offsetlithography, patch coating, screen printing, or transfer from a donorsheet. Alternatively, the patterned polymeric inhibitor layer can beuniformly deposited and patterned using photolithography. The surface ofthe substrate can be optionally treated prior to printing the patternedinhibitor material layer in Step 15.

In step 30, an inorganic thin film layer is deposited by ALD. Thepatterned polymeric inhibitor layer inhibits the deposition of theinorganic thin film material so that the inorganic material onlydeposits on the areas on the substrate where the inhibitor is notpresent. Preferably, there is no measureable deposition of the inorganicmaterial on the areas of the substrate where the inhibitor is present.As such, the inorganic thin film layer has the inverse pattern to thatof the patterned inhibitor layer. Generally, this deposition can be doneusing any ALD system, preferably a spatial ALD system. The inorganicthin film layer can be any material that can be deposited via ALD andwhose growth is inhibited by the inhibitor layer. The inorganic thinfilm layer can be any thickness so long as it has the propertiesnecessary in the final application. Preferred thickness of the inorganicthin film layer is between 5 nm and 100 nm, with thicknesses of lessthan 25 nm being suitable for many applications. The inorganic thin filmlayer is preferably a metal oxide, or a doped metal oxide. Aparticularly useful material for applications requiring a dielectriclayer is Al₂O₃. ZnO is another preferred material in applicationsrequiring semiconductor layer; AZO (aluminum-doped zinc oxide) is apreferred material in applications requiring a conductive layer. Theresult of Step 30 is a patterned inorganic thin film.

In Step 35, the surface of the patterned polymeric inhibitor layer isoptionally treated. The treatment can occur locally, however preferredtreatment methods act uniformly on the substrate surface including thepatterned polymeric inhibitor layer and the patterned inorganic thinfilm layer. The treatment can be understood from the previous discussionand can include using UV-ozone or oxygen plasma processes. The optionaltreatment in Step 35 is necessary when using an ALD process to over coatthe polymeric inhibitor layer. In processes where the polymericinhibitor layer is covered by another polymer layer the optionaltreatment may not be required.

In step 40, a material layer is deposited over the patterned inorganicthin film and the patterned polymeric inhibitor. The material layer canbe an inorganic layer or a polymeric layer. When the material layer isan inorganic layer it is preferred that the deposition is done using anALD process. When depositing another inorganic thin film using ALD, aselective area deposition process can be used to pattern the thin filmduring the atomic layer deposition process. In these embodiments, priorto depositing the material layer in Step 30, another patterned layer ofdeposition inhibitor can be provided so that the material layer ispatterned as deposited using the combination of SAD and ALD processes.When the material layer deposited in Step 30 is a polymeric layer it ispreferred that the polymer layer is a structural polymer. A structuralpolymer is any polymer that is desired to remain in the final structureand should be understood from the previous descriptions. In someembodiments, Step 30 includes depositing a polymeric thin film. In otherembodiments, Step 30 deposits a polymer film using a coating or printingprocess. The material layer in Step 30 can only cover a portion of thesubstrate as long as it covers at least a portion of both the patternedpolymeric inhibitor layer and the patterned inorganic thin film layer.

The process flow described in FIG. 16 can be better understood throughthe descriptive process build shown in FIGS. 17 a through 17 e. In FIG.17 a, the substrate 200 is provided as in Step 10 of FIG. 16. FIG. 17 billustrates the patterned inhibitor layer 220 which is provided on thesurface of the substrate 200 in Step 15 of FIG. 16. Preferably, thepatterned inhibitor layer 220 is a printed patterned inhibitor layer. Asdiscussed above, the surface of the substrate 200 can be treated priorto printing the patterned inhibitor material layer 220. Patternedinhibitor layer 220 contains regions 230 where the inhibitor material isnot present.

FIG. 17 c shows the patterned inorganic thin film material 240 after thesubstrate surface has been exposed to an ALD coating process such thatthe inorganic thin film material is only deposited in the regions 230where the inhibitor material is not present (Step 30 of FIG. 16). FIG.17 d illustrates the resultant structure after depositing material layer250 on the patterned inorganic thin film and patterned inhibitor layer220. This process is particularly useful for forming multilayerinsulating structures. As discussed in the specific transistorembodiments above, the material layer 250 can be a dielectric materiallayer when forming a multilayer insulating structure. In embodimentswhere the material layer 250 is a polymeric dielectric, it is preferredthat the material layer 250 is deposited using a coating process orprinting process. In embodiments where the material layer is aninorganic material layer it is preferred that the deposition be doneusing a chemical vapor deposition process, preferably an ALD process. Itshould be understood from the previous descriptions this method isuseful in forming the patterned polymer interface control layer usefulin top gate embodiments of the present invention. In these embodiments,the material layer 250 is a semiconductor layer and the patternedinorganic thin film material 240 is an electrically conductive material(with the appropriately chosen pattern for the patterned inhibitor layer220).

EXAMPLES General Conditions for the Preparation of Layers UsingAtmospheric Pressure ALD

The preparation of a thin film coating of the material layers on glasssubstrates as used in the examples is described below. The ALD coatingdevice used to prepare these layers, namely aluminum oxide, ZnO:N, andAl-doped ZnO (AZO), has been described in detail in US PatentApplication Publication No. US 2009/0130858, the disclosure of which isherein incorporated by reference in its entirety. The coating device hasan output face (facing up) that contains spatially separated elongatedgas channels and operates on a gas bearing principle. The coating devicecan be understood with respect to delivery head 900 shown in FIG. 18.Each gas channel is composed of an output slot 95, 93, 92 which suppliesgas to the output face 85, and adjacent exhaust slots 91 which removegas from the output face 905. The order of the gas channels isP-O-P-M-P-O-P-M-P-O-P where P represents a purge channel, O represents achannel containing an oxygen based precursor, and M represents a channelcontaining a metal based precursor. As a substrate moves relative to thecoating head it sees the above sequence of gases which results in ALDdeposition.

A 2.5×2.5 inch square (62.5 mm square) glass substrate attached to aheated backer is positioned above the output face of the coating deviceand is maintained in close proximity to the output face by anequilibrium between the pull of gravity, the flow of the gases suppliedto the output face, and a slight amount of vacuum produced at theexhaust slot. For all of the examples, the exhaust slot pressure wasapproximately 40 inches of water below atmospheric pressure. The purgegas P is composed of pure nitrogen. The oxygen reactive precursor O is amixture of nitrogen, water vapor, and optionally ammonia vapor. Themetal reactive precursor M is one or a mixture of active metal alkylsvapor in nitrogen.

The metal alkyl precursors used in these examples were dimethylaluminumisopropoxide (DMAI) and diethyl zinc (DEZ). The flow rate of the activemetal alkyl vapor was controlled by bubbling nitrogen through the pureliquid precursor contained in an airtight bubbler by means of individualmass flow control meters. This saturated stream of metal alkyl was mixedwith a dilution flow before being supplied to the coating device. Theflow of water vapor was controlled by adjusting the bubbling rate ofnitrogen passed through pure water in a bubbler. This saturated streamof water vapor was mixed with a dilution flow before being supplied tothe coating device. The flow of ammonia vapor was controlled by passingpure ammonia vapor from a compressed fluid tank through a mass flowcontroller and mixing with the water vapor stream. All bubblers wereheld at room temperature. The temperature of the coating was establishedby controlling heating of both the coating device and the backer to adesired temperature. Experimentally, the flow rates of the individualgasses were adjusted to the settings shown in Table 1 for each of thematerial layers coated in the examples contained herein. The flows shownare the total flows supplied to the coating device, and thus arepartitioned equally among the individual gas channels.

The coating process was then initiated by oscillating the substrateacross the coating head for the number of cycles necessary to obtain auniform deposited film of the desired thickness for the given example.The coating head as described above contains two full ALD cycles (twooxygen and two metal exposures per single direction pass over the head),therefore a round trip oscillation represents 4 ALD cycles. All sampleswere coated at a substrate temperature of 200° C.

TABLE 1 DMAI DEZ NH3 Water TMA N₂ dilution N₂ dilution N₂ Inert flowflow flow flow flow with Metal with water Purge Residence Layer (sccm)(sccm) (sccm) (sccm) (sccm) Alkyl (sccm) (sccm) (sccm) Time (ms) Al₂O₃65 0 0 65 0 1500 2250 3000 50 ZnO: N 0 60 4 45 0 1500 2250 3000 50 AZO10 30 0 22.5 0 1500 2250 3000 50

Bottom Gate TFT Experiments to Probe the Effect of Passivation

In order to probe the impact of the multilayer insulating structure ontransistor performance and stability, bottom-gate TFTs were built usingthe spatial atomic layer deposition tool described above. The ExampleTFTs were fabricated as described below, and the relative differencesbetween the samples are called out in the following descriptions forclarity. Electrical testing of the transistors was accomplished by usinga probe station to contact the gate and source/drain electrodes. Thetransistors were swept in the linear regime, with the drain being heldconstant at 0.2V (Vd=0.2), and the gate voltage was swept over the rangeindicated. The mobility (Mob), threshold voltage (Vth), drain current atmaximum gate bias (Ion), the current flowing at Vg=0, and average gateleakage current at the maximum gate voltage applied (Ig(vgmax)) wereextracted from the measurements taken. One aspect of the inventionrelates to stable enhancement mode transistors, so it is desirable thatthe Ion be high, and the current flowing when there is no bias on thegate (Vg=0) be as low as possible. It is further desired that theperformance of the transistor remains unchanged under bias stress.

Comparative Example C1 Bottom Gate TFT with PVP Deposition InhibitorPassivation on a Glass Substrate

Comparative Example C1 is a bottom-gate device having a simple polymerin contact with the backchannel of the TFT. Comparative Example C1 wasfabricated using the combination of spatial ALD and selective areadeposition (SAD). The gate layer was 1000 Å of AZO, the gate dielectricwas 500 Å of Al₂O₃, 200 Å of ZnO:N was used for the semiconductor layerand 1000 Å AZO was used for the source and drain electrodes. The printedinhibitor used to pattern each of the device layers was a 2 wt %polyvinyl pyrrolidone (PVP) k30 in diacetone alcohol solution which wasprinted using a Dimatix 2500 printer. The pattern of PVP used to patternthe AZO layer in to the source and drain was left on the device duringtesting, such that the back channel of the semiconductor layer wascovered by a thin layer of PVP k30. The transistor dimensions were W=400microns, and L=95 microns. The extracted performance values forComparative Example C1 can be found in Table 2.

Comparative Example C2 Bottom Gate TFT with NO Passivation on a GlassSubstrate

Comparative Example C2 is a bottom gate TFT which had all materials thatwere in contact with the semiconductor back channel removed. ComparativeExample C2 was fabricated like Comparative Example C1 using thecombination of spatial ALD and selective area deposition (SAD), and wasformed from the same material stack and transistor geometry. The PVPpattern used to pattern the AZO layer in to the source and drain wasremoved from the device using a 100 W 0.3 Torr oxygen plasma, in aTechnics PEIIA parallel plate system. Prior to removing the PVP from theback channel, the behavior of Comparative Example C2 measured and foundto be similar to that of Comparative Example C1. However, removing thepolymer from the back channel caused the transistor behavior to change,with a negative shift in Vth of 3.6 volts. The characteristicperformance parameters for Comparative Example C2 can be found in Table2.

Comparative Example C3 Bottom Gate TFT with Al2O3 Passivation on a GlassSubstrate

Comparative Example C3 was fabricated as Comparative Example C2 with thefollowing exceptions. After removing the PVP used to pattern the sourceand drain electrodes, a 750 Å thick aluminum oxide passivation layer wasdeposited using the SALD equipment described above with 2312 ALD cyclesand DMAI as the precursor. The passivation layer was patterned using aPVP k30 printed inhibitor in order to provide access to the source,drain and gate electrodes for electrical testing. The deposition ofalumina on the back channel resulted in a transistor with a negativethreshold voltage. The characteristic performance parameters forComparative Example C3 can be found in Table 2; a visualization of theperformance can be gained from the Id−Vg curve shown in FIG. 20.

Inventive Example I1 Bottom Gate TFT with Only SU-8+Al2O3 Passivation ona Glass Substrate

Inventive Example I1 is a bottom-gate transistor that was passivatedwith a multilayer dielectric stack of the present invention. InventiveExample I1 was fabricated as Comparative Example C2 with the followingexceptions. After removing the PVP used to pattern the source and drainelectrodes, the transistor was passivated using a multilayer dielectricstack of the present invention. The multilayer dielectric stack wasformed by spinning on a 10% solution of SU-8 2010 in PGMEA. The SU-8 wasblanket exposed and cured at 225° C., resulting in a cured layer of SU-8with a nominal thickness of 1000 Å. After curing the SU-8, the surfacewas treated with a 30 second 100 W 0.3 Torr oxygen plasma to enable thegrowth of Al₂O₃ on the SU-8 surface. A 2% solution of PVP k30 wasprinted on the SU-8 surface to define the pattern for the inorganiclayer of the multilayer insulating structure. The sample was then coatedwith 250 Å of Al₂O₃ using the SALD system described above with DMAI asthe precursor at 200° C.; the PVP inhibited the growth such that Al₂O₃was only deposited in the areas of the SU-8 that did not have PVP. TheSU-8 was then patterned using a 2 minute oxygen plasma at 300 W and 0.4Torr, which removed the PVP inhibitor and the SU-8 that was notprotected by the patterned Al₂O₃, resulting in a polymer structure withpattern having an area that lies within the pattern of the inorganiclayer. The transistors were then characterized as for the ComparativeExamples, and the performance data for Inventive Example I1 can be foundin Table 2; a visualization of the performance can be gained from theId−Vg curve shown in FIG. 20.

TABLE 2 Passivation N Ion Ioff Sample Layer TFT Mob Vth Von (Vg = 14 V)(Vg = 0 V) C1 PVP k30 19 17.22 5.51 −0.71 1.53E−5 1.51E−9 (1.41) (1.07)(0.65) (1.61e−6) C2 None 1 21.45 2.60 −5.83  2.9E−5 4.18E−7 C3 Al2O3 2921.15 −0.98 −4.88 3.65E−5 3.48E−6 (1.66) (0.49) I1 SU-8/Al2O3 70 17.222.91 −0.61 2.26E−5  4.61E−10 (2.27) (0.85) (0.59)

As can be seen in Table 2, removing the polymer from the backchannel inComparative Examples C2 causes the threshold voltage to shift around −3volts, while shifting the turn-on voltage by −5 V resulting in a poorsub-threshold slope. When the polymer used to pattern the source anddrain electrode was removed and the back channel was passivated withonly Al₂O₃, both the turn-on and the threshold voltage are negativeresulting in a depletion mode TFT as seen by the data for ComparativeExample C3. The negative threshold voltage shift associated withinorganic passivation causes the TFT to be fully on at 0 V, making itineffective as a switch when applying a positive voltage bias. Ifinstead, after removing the polymer used to pattern the source and drainelectrode, the back channel is passivated with the multilayer insulatingstructure of the present invention, there is a shift in the thresholdvoltage with no change in the turn-on voltage resulting in the moredesirable enhancement mode TFT as seen in the data for Inventive ExampleI1.

These samples were then evaluated for stability under stress. Forcomparison, each sample was operated at a constant drain current of 10uA with a constant gate voltage of 14 V. Each sample was held underconstant stress, which was periodically stopped (every 400 seconds) andthen the device was swept in the linear regime with Vd=0.2 V. Thetransistor performance after 10 hours of stress can be found in Table 3,and the change in threshold voltage as a function of stress time isshown in FIG. 19.

TABLE 3 Passivation Ion Ioff Delta Sample Layer Mob Vth Von (Vg = 14 V)(Vg = 0 V) Vth Id/Io C1 PVP k30 14.46 6.98 0.87 1.25E−05 1.41E−10 1.220.88 C2 None 23.63 7.46 −6.33 1.89E−05 9.52E−08 4.86 0.65 C3 Al2O3 20.24−0.42 −5.35  3.5E−05 2.98E−06 0.41 0.98 I1 SU-8/Al2O3 18.13 4.22 −0.082.19E−05 2.31E−09 0.08 1.01

As seen in Table 3, Inventive Example I1 had the smallest thresholdvoltage shift, and thus performed the best, under bias testing.Comparative example C2, which had an unprotected back channel had alarge threshold voltage shift with only 10 hours of bias testing, aswell as a decreased Ion current and a higher-than-desirable off current(Vg=0). Comparative example C1 with only PVP on the backchannelperformed better than comparative example C2. However, PVP is not anenvironmental barrier, so is insufficient as a final passivation andbarrier layer. Comparative Example C3 with only alumina passivation wasstable under stress, however the device is always on and is not anenhancement mode transistor. In comparison, Inventive Example I1 hadstable on current, a stable threshold voltage and stable turn-on voltageposition, indicating that the device is a stable enhancement modedevice. FIG. 20 clearly illustrates the difference in performancebetween Inventive Example I1, an enhancement mode device and ComparativeExample C3, a depletion mode device.

Vertical TFT Experiments to Probe the Effect on the Back Channel Surface

In order to probe the impact of the multilayer insulating structure ontransistor performance and stability, vertical TFTs were fabricated.Electrical testing of the vertical transistors was accomplished by usinga probe station to contact the gate and source/drain electrodes, and thedata was evaluated in the same manner as with the bottom gatetransistors.

Inventive Example I2 Vertical TFT with Only SU-8+Al2O3 Passivation on aGlass Substrate

Inventive Example I2 is a vertical transistor that was passivated with amultilayer dielectric stack of the present invention. The transistor wasfabricated using the combination of spatial ALD and selective areadeposition (SAD). The gate structure was formed by coating 6000 Å layerof cured SU-8 by spinning a 33.5% solution of SU-8 2010 in PGMEA at 3000rpm, blanket exposing, and baking with a final hard bake for 5 minutesat 225° C. A hard mask of Al₂O₃ was patterned using selective areadeposition with an inkjet printed PVP inhibitor mask. The gate structurewas completed by etching the PVP and SU-8 for 6 minutes in a 300 W, 0.4Torr oxygen plasma to form a reentrant profile, and then uniformlycoating with 250 Å of Al₂O₃. A gate layer of 1000 Å AZO was depositedover the gate structure, the gate dielectric was 400 Å of Al₂O₃, thesemiconductor layer was 200 Å of ZnO:N, and the source and drainelectrodes were 1000 Å of AZO. The inhibitor used to pattern each of thedevice layers was a 2 wt % PVP in diacetone alcohol solution, which wasprinted with a Fuji Dimatix printer. The transistor dimensions were W254 microns, with an approximate length L=17 microns. The PVP patternused to pattern the AZO layer in to the source and drain was left on thedevice during initial characterization, such that the back channel ofthe semiconductor layer was covered by a thin layer of PVP k30. Aftercharacterization, the PVP was removed from the semiconductor surfaceusing a 2 minute 100 W, 0.3 Torr oxygen plasma.

Next, the sample was passivated by forming the multilayer dielectricstack. The multilayer dielectric stack was formed by spinning on a 33.5%solution of SU-8 2010 in PGMEA. The SU-8 was blanket exposed and finalcured at 225 C, resulting in a cured layer of SU-8 with a nominalthickness of 6000 A, with the SU-8 filling the reentrant profile of thevertical transistor. After curing the SU-8, the surface was treated witha 30 second 100 W 0.3 Torr oxygen plasma to enable the growth of Al₂O₃on the SU-8 surface. A 2% solution of PVP was printed on the SU-8surface to define the passivation pattern. The sample was then coated at200° C. with 250 Å of Al₂O₃ using the SALD system described above withDMAI as the precursor; the PVP inhibits the growth such that Al₂O₃ wasonly deposited in the areas of the SU-8 that did not have PVP. The SU-8was then patterned using a 6 minute oxygen plasma at 300 W and 0.4 Torr,which removed the PVP inhibitor and the SU-8 that was not protected bythe patterned Al₂O₃. The transistors on the sample were thencharacterized in the same manner as the original sample, and showed nothreshold voltage shift after passivation. The initial value and thechange with passivation for a representative transistor for InventiveExample I2 can be found in Table 4, as well as in FIG. 21.

Inventive Example I3 Vertical TFT PVP+SU-8+Al2O3 Passivation on a GlassSubstrate

Inventive Example I3 is vertical transistor that was passivated with analternative multilayer dielectric stack of the present invention.Inventive Example I3 was fabricated and tested using the same processsteps as Inventive Example I2, with the following exception. Afterinitial characterization of Inventive Example I3, the PVP inhibitor wasnot removed—instead the PVP was left in place and coated with the 6000 Åof SU-8 during the passivation process. As such, the multilayerdielectric stack of Inventive Example I3 includes the PVP (which wasoriginally used as the deposition inhibitor for patterning the sourceand drain), the SU-8, and Al₂O₃. The data from the characterizationtesting for a representative transistor, including the initial valuesand the change with passivation for Inventive Example I3 can be found inTable 4, as well as in FIG. 22.

TABLE 4 Before Passivation After Passivation Passivation Channel ChannelInitial Initial After After Sample Layer Width Len N TFT Mob Vth Mob Vth12 SU-8/Al2O3 254 17 48 14.4 3.4 8.9 2.9 13 PVP SU-8/Al2O3 185 28 60 8.73.7 68 3.7

The VTFTs had no statistical shift in threshold voltage afterpassivation, although there was a small decrease in both the mobilityand the on current calculated from a linear gate voltage sweep (Vd=0.2).This response to passivation was independent of channel length andwidth, and was similar for both I2 and I3 illustrating that bothembodiments of the multilayer dielectric structure maintain the originaltransistor performance. To evaluate the effectiveness of the multilayerdielectric stack as a passivation layer for devices under stress,representative transistors each from Inventive Examples I2 and 13 wereoperated at a constant current of 10 uA. Each sample was held understress for 24 hours with a gate voltage of 8 V with a drain current of10 uA. Periodically, every 400 seconds, the transistor was swept in thelinear regime, with the gate voltage swept from −2 to 8 V and Vd held at0.2 V. The results of this testing can be found in Table 5.

TABLE 5 Passivation Channel Channel Sample Layer Width Len Initial VthIo Delta Vth Id/Io 12 SU-8/Al2O3 254 17 3.3 2.0E−05 −0.70 0.80 13 PVPSU-8/Al2O3 185 28 3.6 7.0E−06 −0.42 1.03

As can be seen from Table 5, inventive examples I2 and I3 had stable oncurrent values and threshold voltage position. FIG. 21 has the data forfour linear sweeps for Inventive Example I2 that are nearly identical;these measurements were taken after fabrication (before passivation),after passivating with the multilayer insulating structure, afterpassivation and immediately before applying the stress conditions, andafter 24 hours of constant current stress. Similarly, FIG. 22 has thedata for four linear sweeps for Inventive Example I3 that are nearlyidentical; these measurements were taken after fabrication (beforepassivation), after passivating with the multilayer insulatingstructure, after passivation and immediately before applying the stressconditions, and after 24 hours of constant current stress. It is clearfrom the data in Table 5 and FIGS. 21 and 22 that the multilayerinsulting structure is an effective passivation layer for the verticaltransistor geometry.

Top-Gate TFT Experiments to Probe the Effect Back Channel Surface

In order to probe the impact of the using a polymer charge-control layeron the back channel of a top gate TFT, a number of top-gate devices werefabricated. Electrical testing of the top-gate transistors wasaccomplished by using a probe station to contact the gate andsource/drain electrodes, and the data was evaluated in the same manneras with the bottom-gate transistors. The transistors were swept in thelinear regime, with the drain being held constant at 0.2 V (Vd=0.2), andin the saturation regime, with the drain being held constant at 15V or20 V depending on the device. The mobility (Mob), threshold voltage(Vth), and the turn-on voltage (gate voltage for 1 nA of drain current,Von) were extracted from the measurements taken. One aspect of theinvention relates to stable enhancement mode transistors, so it isdesirable that the Ion be high, Vth be positive, and Von be close tozero.

Comparative Example C4 Top Gate TFT on a Glass Substrate

Comparative Example C4 was fabricated on a glass substrate, such thatthe back channel of the semiconductor layer was in contact with theglass. The transistor tested was a top-gate device that was fabricatedusing the combination of spatial ALD and selective area deposition(SAD). The layers were deposited using the conditions above andpatterned with an inkjet-printed inhibitor in the following order: 200 Åof ZnO:N semiconductor layer, 1000 Å AZO source and drain layer, 750 ÅAl₂O₃ gate dielectric, and 1000 Å of AZO for the gate. The printedinhibitor used to pattern each of the device layers was a 2 wt % PVP indiacetone alcohol solution which was printed using a Fuji Dimatixprinter. The transistor dimensions were W=400 microns, and L=95 microns.The data for a representative transistor for Comparative Example C4 canbe found in Table 6, with Vd=15 V used for the saturation sweep. Thesaturation Id−Vg curve for Comparative Example C4 can be found in FIG.23.

Comparative Example C5 Top Gate TFT on a Glass Substrate withUnpatterned SU-8

Comparative Example C5 was prepared like Comparative Example C4 with thefollowing exceptions. Prior to depositing the semiconductor layer theglass was coated with a uniform SU-8 layer by spin coating a 10%solution of SU-8 2010 in PGMEA and curing at 225° C. for 5 minutes. Thenominal layer thickness was 1000 Å. The SU-8 was then treated with a 3second, 100 W 0.3 Torr oxygen plasma to promote the SALD growth on theSU-8 surface. The sample was then completed as for Comparative ExampleC4 and analyzed, with Vd=20 V used for the saturation sweep. The resultsof this testing can be found in Table 6; the saturation Id−Vg curve forComparative Example C5 can be found in FIG. 23.

Inventive Example 14 Top Gate TFT on a Glass Substrate with PatternedSU-8

Inventive example I4 was prepared as Comparative Example C5 with thefollowing exceptions. After depositing the semiconductor layer over theSU-8 layer, the SU-8 that was not under the semiconductor was removed.The removal was done using a 300 W, 0.4 Torr oxygen plasma for 3 minuteswhich removed the PVP layer used to pattern the ZnO:N, and the SU-8layer not protected by the semiconductor. The sample was then completedas for Comparative Example C5 and analyzed, with Vd=20 V used for thesaturation sweep. The results of this testing can be found in Table 6.

TABLE 6 Linear Linear Linear Sat Sat Sat Sample Mob Vth Von Mob Vth VonC4 4.23 4.19 −10.85 2.88 0.03 −12.41 C5 0.63 11.90 0.30 1.02 5.70 −0.54I4 0.18 11.36 1.15 0.62 5.23 0.06

As can be seen in from Table 6, while the calculated mobility ofComparative Example C4 was higher than the mobility for ComparativeExample C5 and Inventive Example I4, the transistors grown directly onthe inorganic substrate did not turn off at gate voltage of 0 V. Incontrast, the transistors whose semiconductor was deposited on polymer,Comparative Example C5 and Inventive Example I4, exhibit reasonableenhancement mode performance with turn on voltages near zero. A visualcomparison of the relative device performance gained from examination ofthe Id−Vg curves shown in FIG. 23.

The invention has been described in detail with particular reference tocertain example embodiments thereof, but it will be understood thatvariations and modifications can be effected within the scope of theinvention.

PARTS LIST

-   -   10 providing a substrate    -   15 providing a patterned polymeric inhibitor    -   30 depositing an inorganic thin film by ALD    -   35 optionally treating the surface    -   40 depositing a material layer    -   91 exhaust channels    -   92 metal precursor flow    -   93 oxidizer-containing flow    -   95 nitrogen purge flow    -   97 example substrate    -   98 arrow    -   99 gap    -   101 transistor    -   103 transistor    -   104 transistor    -   105 transistor    -   106 transistor    -   107 dual gate transistor    -   108 vertical transistor    -   109 vertical transistor    -   110 substrate    -   111 vertical transistor    -   112 transistor    -   116 transistor    -   120 gate    -   125 second gate    -   130 patterned insulating layer    -   150 patterned semiconductor layer    -   160 source    -   165 drain    -   170 polymer structure    -   171 polymer layer    -   175 polymer layer    -   178 polymer structure    -   179 polymer layer    -   180 inorganic thin film dielectric layer    -   185 second inorganic thin film dielectric layer    -   188 inorganic thin film dielectric layer    -   189 second inorganic thin film dielectric layer    -   190 multilayer insulating structure    -   194 passivation layer    -   195 multilayer insulating structure    -   200 substrate    -   208 vertical transistor    -   220 patterned inhibitor layer    -   230 regions where inhibitor material is not present    -   240 patterned inorganic thin film material    -   250 material layer    -   470 polymer structure    -   500 top gate transistor    -   505 top gate transistor    -   510 substrate    -   515 top gate transistor    -   520 gate    -   530 patterned insulating layer    -   550 patterned semiconductor layer    -   560 source    -   565 drain    -   570 polymer layer    -   575 patterned polymer layer    -   578 patterned polymer layer    -   700 a,b,c width of top    -   705 a,b,c minimum width defining reentrant profile    -   710 a,b,c first side    -   720 a,b,c second side    -   730 a,b,c height of gate structure 820    -   735 a,b,c top electrically conductive gate structure    -   778 polymer layer    -   810 substrate    -   820 electrically conductive gate structure    -   840 a,b,c reentrant profile    -   845 a,b,c reentrant profile    -   850 patterned insulating layer    -   860 patterned semiconductor layer    -   870 first electrode    -   875 second electrode    -   880 third electrode    -   890 multilayer insulating structure    -   900 delivery head    -   905 output face    -   A,A′ cross section line    -   P,P′ line defining reentrant profile

1. A transistor comprising: a substrate; a gate in contact with thesubstrate; a gate insulating layer in contact with at least the gate; aninorganic semiconductor layer in contact with the gate insulating layer;a source electrode in contact with a first portion of the inorganicsemiconductor layer; a drain electrode in contact with a second portionof the inorganic semiconductor layer, the source electrode and the drainelectrode separated by a gap; and a multilayer insulating structure incontact with at least the inorganic semiconductor layer in the gap, themultilayer structure including: an inorganic dielectric layer having afirst pattern defining a first area; and a multilayer polymer structurecomprising: a first polymeric layer having a second pattern definingsecond area located within the first area; and a second polymeric layerin contact with the first polymeric layer and with the semiconductorlayer in the gap, the second polymeric layer having a third patterndefining a third area located within the second area, wherein the thirdarea is smaller than the second area.
 2. (canceled)
 3. The transistor ofclaim 1, wherein the first polymeric layer is not in contact with thesemiconductor layer in the gap.
 4. The transistor of claim 1, furthercomprising another inorganic dielectric layer that is conformal andextends beyond the first area.
 5. A transistor comprising: a substrate;a gate in contact with the substrate; a gate insulating layer in contactwith at least the gate; an inorganic semiconductor layer in contact withthe gate insulating layer; a source electrode in contact with a firstportion of the inorganic semiconductor layer; a drain electrode incontact with a second portion of the inorganic semiconductor layer, thesource electrode and the drain electrode separated by a gap; and amultilayer insulating structure in contact with at least the inorganicsemiconductor layer in the gap, the multilayer structure including: aninorganic dielectric layer having a first pattern defining a first area;and a multilayer polymer structure comprising: a first polymeric layerhaving a second pattern defining second area located within the firstarea; and a second polymeric layer in contact with the first polymericlayer and with the semiconductor layer in the gap, the second polymericlayer having a third pattern defining a third area located within thesecond area, wherein the first area extends beyond the second area todefine a reentrant profile under the inorganic dielectric layer, thetransistor further comprising another inorganic dielectric layer that isconformal and extends beyond the first area. 6.-8. (canceled)
 9. Thetransistor of claim 1, wherein the first area is equal to the secondarea.
 10. The transistor of claim 1, wherein the multilayer insulatingstructure is in contact with a portion of the source electrode and aportion of the drain electrode.
 11. The transistor of claim 1, whereinthe transistor is an enhancement mode device.
 12. The transistor ofclaim 1, wherein the inorganic semiconductor is a metal oxidesemiconductor.
 13. The transistor of claim 12, wherein the metal oxidesemiconductor is a ZnO based semiconductor.
 14. The transistor of claim1, the gate being a first gate, further comprising: a second gatelocated on the multilayer insulating structure on a side of themultilayer insulating structure opposite the first gate, and over atleast a portion of the gap.
 15. The transistor of claim 14, wherein thefirst gate and the second gate are aligned relative to each other. 16.The transistor of claim 14, wherein the first gate and the second gateoverlap each other in the gap.